XPS 420, go from Vista to clean install Windows 10?

No you cannot install windows 10 at all. Vista computers are too old to run windows 10 successfully. There will not be any drivers for it. Also, if you are running 32 bits windows now, your processor is probably not capable of running 64 bits.

3 GB of RAM is low for Win 10. And a PC that has Vista certainly doesn't use DDR6 RAM, assuming DDR6 RAM actually existed, aside from on video cards (GDDR6).

If your PC supports 64-bit Win 10, you can install it directly without any intermediate versions. Keep in mind that there's no free upgrade from Vista to Win 10 so you will have to buy a license from Microsoft to activate Win 10 on this PC. And if this PC came with Vista, it's unlikely that Dell supports it for Win 10...

EDIT: @Mary G typed faster than I did...as usual!

There are no updates from 32 to 64 or 64 to 32.

The key does not care about the media 32 or 64 only the machine ID and Activation.

Once you have activated 10 you can clean install 32 or 64 with the  "I don't have a key" option.

Once you go online it will recognize that you have previously activated.

There are ZERO updates to 10 from XP or Vista.

"you cannot install windows 10 at all. Vista computers are too old to run windows 10 successfully."

All machines with at least WDDM 1.0 vista drivers will work with windows 10.  Its not an upgrade but rather a new clean install of OEM system builder or retail windows. A Vista Capable or equivalent PC is capable of running all editions of Windows Vista and has WDDM 1.0 or newer drivers although some of the special features and high-end graphics options may require at least GT1030 GPU and 4 gigs or more ram. 

The machine in this video is a 2006 XPS 400/Dimension 9150

https://www.youtube.com/watch?v=qa9b0TkTsLM

The reason to not do this is because you can get an older machine that already has windows 10 installed for LESS than the cost of the windows licence being $119 for home and $199 for Pro.   OEM versions are $109 or $179 in that range.

Dells that are from the "Vista Capable era aka they have "AT LEAST" pentium D 900 series Pressler CPU will run windows 10.

Earlier Pentium D 800 series Smithfield will only run 32 bit versions of Windows 8.1 or 10.

Windows XP/VISTA/7/8.0 are all end of life end of support.

That means systems as old as Optiplex GX620/XPS 400/Precision 380 etc WILL RUN windows 10 but they are not supported.

Drivers for Windows 10 are the vista or win7 WDDM 1.0 or newer drivers and work just fine.

I call these older systems "Free computers" because they cost less than buying a legit copy of windows 10.

https://www.lambroinc.com/desktops?lightbox=dataItem-jtfsw8uq

A machine like GX620 with at least 3 gigs of ram that has PCI-E video slot works fine with 10 and a GT1030 video card.

Buying SSD and GT1030 isn't a waste of money because they can be used in newer machines.

Buying DDR2 or DDR3 ram or an older cpu if it costs more than a few dollars is a waste.

Vista systems are 2007-2009 models making them 11-13 years old. The later systems are Windows 10 64 Bit capable but you will have borderline performance. Many of the earlier systems had 32 Bit only processors and are below minimum requirements for Windows 8.1 64 Bit or Windows 10 64 Bit.

Also there is no Free Upgrade for Windows Vista to Windows 10* and a Full Retail License will cost more than the hardware is worth. You can however install Windows 10 without a Product Key to run it unlicensed. Unlicensed Windows 10 will be watermarked and some personalisation settings will be grayed out but everything else should work.

* Windows Vista Systems released in 2008-2009 such as the OptiPlex 760 were eligible for a Free Upgrade to Windows 7 and had a BIOS Update which makes the system eligible for Dell Windows 7 OEM SLP. This can be used to get free Windows 10.

You seem to have confusion about RAM and Disc Space. DDR, DDR2, DDR3, DDR4, DDR5, DDR6 RAM modules are not cross compatible. Most Windows Vista systems came with DDR2 but the pin out will differ if it is a laptop and a desktop. If you have DDR RAM then the system is ancient and incompatible with Windows 10. You should ideally have at least 4 GB DDR2 of RAM to run Windows 10. Most Windows Vista systems had a SATA  slot for a Drive and if you are spending money on upgrades you can get a cheap 240 GB Solid State Drive.

If you want more specific details, provide details about your system. Press the [Windows] and [ r ] key and type in msinfo32 and then press [Enter]. Provide the System Model, Processor, BIOS Version/Date, SMBIOS Version and Installed Physical Memory (RAM).

Dear Dr. Yip,

I couldn't find a thread for reinstalling Windows on an Inspiron 9400, so I am posting here in hopes you will see this!

The Inspiron 9400 has been a great laptop for me for many years, starting with XP.  Thanks to your comprehensive reinstallation guide I was able to get Windows 7 installed.

Then a couple of years ago I decided to go for Windows 10, and after great difficulty finding usable drivers succeeded!  Even with a 1.67 GHz T2300 CPU and only 1.5 GB of RAM, Windows 10 runs nicely on the 17" 1920x1200 display (ATI Mobility Radeon X1400).

Last month the new Windows 10 "2004" (19041.264) came out so I unthinkingly installed it.  That broke the video graphics: moving the cursor across a window boundary caused a (temporary) fast flicker, and overlapping windows were pieced together, or not restored when I clicked on the title bar of the back window, and/or interleaved with some sort of "screen door" alpha.

Fortunately I was able to roll back from "2004" to "1909" (18363.836) and now Windows 10 works again, but that was a close call, so I wanted to share it with you. 

Do not update an Inspiron 9400 to Windows 10 "2004" (19041.264)!

Thanks!

Inspiron 9400 from 2004 lacks basic hardware to work with Windows 8 let alone windows 10.

You cannot install 8 or 10 on an inspiron 9400 because it does not support

• Processor. 1 gigahertz (GHz)* with support for PAE, NX, SSE2, LAHF, SAHF, CMPXCHG16
• RAM. 2 gigabyte (GB) (32-bit) or 4 GB (64-bit)
• Hard disk space. 32 GB (32-bit) or 64 GB (64-bit)
• Graphics card. Microsoft DirectX 9 graphics device with WDDM driver

CMPXCHG16 is REQUIRED AS IS NX

Yes, I installed Windows 10 on a Dell Inspiron 9400.

Here is the screenshot:

http://courses.cse.tamu.edu/daugher/IMG_2033.jpg  

What's that old saying?  "They said it couldn't be done, but I did it!"

Cheers!

P.S.  Photo is now at https://people.engr.tamu.edu/daugher/IMG_2033.jpg