Ok thanks!  :-)

Dell usually only sell the OEM version of Windows complete with hardware, they do not sell the OEM license separately.

They also sell upgrade discs which are the exact same as the retail upgrade discs which you can get from a store or from the likes of Amazon. However right now the only retail media they are selling is for Windows 8.1. Even if you buy this media from Dell, you won't get any additional support from Dell on an OS they deem unsupported on that model. The only difference is it will likely cost you more for the upgrade license direct from Dell than from elsewhere. Thus there is no added benefit buying upgrade media direct from Dell. Its an area I think Dell are missing out on. As mentioned also Dell have also stopped selling Windows 7 Upgrade licenses.

Once you have bought a store license of Windows 7 follow Windows Reinstallation Guide/A Clean Install of Windows 7:

http://philipyip.wordpress.com/dell-community-forums/ 

This will guide you through the installation process. Dell do not provide Windows 7 drivers for this model, however I have compiled a set here from newer systems with the same hardware components and direct from vendors in order to assist end users to prepare for Windows XP End of Life:

http://philipyip.wordpress.com/unofficial-driver-sets/inspiron-laptops-windows-7/inspiron-1520-windows-7-64-bit/

http://philipyip.wordpress.com/unofficial-driver-sets/inspiron-laptops-windows-7/inspiron-1520-windows-7-32-bit/

Following on - thanks for the useful advice natakuc4.  I'm not very experienced when it comes to messing around with operating systems, so wonder if you could advise me too.  We have 2 Dell laptops both running on Windows XP. I've been reading through the various posts regarding the forthcoming lack of Microsoft support in April 2014 but am still unsure as to what the best way forward for us is.

My laptopis a Dell Inspiron 6400 with 74.5gb hard-drive. My partner's is a Dell Latitude D610 with a 40gb hard-drive. Should we change both to Windows 7? Is there any particular version that would be the most suitable for both laptops? Can I change the operating system myself or should I invest in an expert to do both. I would also be interested to know what other 'beginners' with XP are planning on doing, if anything at all?

Each Windows 7 Home Premium license is going to cost you $100.

Personally, I don't think it is "worth it" to upgrade those old laptops for that much money considering that a brand new laptop is $300 and will be way faster than yours.

That D610 is not going to have much hard drive space left over after Windows 7 since the system requirement is 16GB.

If it was me, I would buy new laptops and install Linux on these laptops and use it as a learning experience.

First can you run the Crucial System scanner on both units? Its important to know whether each system has 1 GB or more RAM installed otherwise the RAM needs to be upgraded to conform to Windows 7 minimum requirements:

http://www.crucial.com/

The Dell Inspiron 6400 runs Windows 7 32 Bit Home Premium quite nicely. I have upgraded several of these to that version of Windows and made an unofficial set of system drivers here:

http://philipyip.wordpress.com/unofficial-driver-sets/inspiron-laptops-windows-7/inspiron-6400-windows-7-32-bit/

I gave one a test on Windows 8.0 32 Bit and it also worked very well. I have never tried it with Windows 8.1 32 Bit but that configuration should also work. Windows 8.1 is in general cheaper and easier to find than Windows 7 these days.

The Windows Reinstallation Guide/A Clean Install of Windows 7/8.1 will instruct you through the clean installation step by step:

http://philipyip.wordpress.com/dell-community-forums/

Note you can install Windows 7 Home Premium using the .iso here:

http://philipyip.wordpress.com/download-microsoft-windows/windows-7-sp1-iso-download/

When prompted for a product key you can select skip which will grant you a 30 day trial which you can use to evaluate how well Windows 7 runs. You can simply buy Windows 7 and activate the product with the product key affixed to the retail installation media.

For Windows 8.1 it is not so easy to trial. There is the enterprize trial which you can download here for a 90 day trial:

http://technet.microsoft.com/en-gb/evalcenter/hh699156.aspx

Unfortunately if you decide to buy Windows 8.1 then its not as simple as inputting the key and activating you will need to reinstall with the media that your purchased.

I would advise installing the trial of either OS before deciding on a purchase. 

The Crucial system scanner will also recommend Solid State Upgrades for the Inspiron 6400. I highly recommend replacing the hard drive with a 120 GB Crucial M500 which have went significantly down in price recently. The performance of an Inspiron 6400 and Windows 7 will feel like a new computer.

Note I also recommend using a USB cooling pad with the Inspiron 6400 to prevent it from overheating (if you haven't got one).

If you are going to purchase Windows 8.1 DO NOT PURCHASE FROM THE MICROSOFT STORE as they will provide you only with a download link which will mean you need your previous version of Windows installed in order to download and install Windows 8.1. This will be a pain if you are going to install the Solid State Drive as it is more difficult to install XP on a SSD (and a waste of time to install XP again). If you purchase through the likes of Amazon or a shop you should get an actual installation DVD which will allow you to install Windows 8.1 directly on a new HDD or SSD.

The Latitude D610 I would not recommend upgrading. The hardware is significantly older for this model. The hard drive for instance is IDE and your one is of low storage capacity. These drives were made obsolete by the SATA interface (and newer versions of SATA III are backwards compatible to SATA II and I). In short the IDE hard drives are no longer manufactured and short in spares and there is pretty much no feasible Solid State Drive Upgrade. Moreover as the hard drive is the most common component to fail I would not recommend pumping money into upgrading this unit in case it does fail. Next the graphics adapter, most of these units were provided with the Intel 915GM which does not quite support Windows 7. The Windows Vista 32 Bit Driver from HP will work with Windows 7 but its overall performance will be quite low. However the system can be made to run Windows 7 32 Bit at a push and I have a set of system drivers here:

http://philipyip.wordpress.com/unofficial-driver-sets/latitude-d-series-laptops/latitude-d610-windows-7-32-bit/

You might want to consider a new system or a newer second hand system opposed to replace the D610. Even the lowest end system Dell are selling for £280 is leaps ahead of the D610:

http://www.dell.com/uk/p/laptops?ref=DHSHP_C1R01_laptops#!everyday-laptops

Dell Outlet may also some interesting details in particularly in the lead up to (and just after) Christmas:

http://outlet.euro.dell.com/Online/InventorySearch.aspx?brandId=7&c=uk&cs=ukdfh1&l=en&s=dfh

If purchasing a new Dell system call Dell Technical Support as soon as it arrives to request a Dell Windows 8.1 Reinstallation DVD. Dell no longer ship these with the system and you are meant to make your own recovery media but this can cause a lot of headaches. The Reinstallation DVD is way more reliable and its best to call Dell about it in the first 30 days.