thanks  for  the  update   well  that  puts me off ever buying  dell  again

upgrade to Windows 19

Don't rush things! :emotion-2:

The 745 isn't on Dell's list of systems tested for compatibility with Win 10. Doesn't mean it won't run, but likely means there can be driver issues.  Note that Dell didn't offer drivers for Win 8 (8.1) for this model either.

You might try running the Win 8.1 upgrade assistant from Win 7 to see what it says. If it reports incompatibility issues with 8/8.1, you'll likely have the same and more issues trying to install Win 10.

You can also search these forums to see if anybody has reported successful upgrading to Win 10.

You can't blame Dell because your old hardware won't run the newest OS.

Win 8/8.1 and 10 have hardware requirements that didn't exist in older PCs like your 745. Win 8/8.1 and Win 10 require a CPU with the NX (no execute) function.  Look in BIOS to see if that option is available. If it is, enable it, and see if Win 10 will install now. (Be sure to back up all personal files on external media first!).

My Dimension doesn't run anything beyond Win 7 because it 'has an old CPU that doesn't support NX. That's not Dell's fault.

Even if your hardware could run the latest OS, it's also possible some software can't handle Win 10. Dell has no control over 3rd party drivers and software.

Run the Win 8.1 upgrade assistant and see what it says. If it's only a matter of a driver(s), and you're willing to poke around on the net, you might find what you need. If it says you'd need a new CPU, it's probably not worth the cost to upgrade an old PC...

The OptiPlex 745 used Core 2 Duo CPUs, which have the NX function and should upgrade to Win 8 and 10..

Did you register for the upgrade with Microsoft, and if so, did you get any message indicating it will upgrade or that it won't because of a specific reason? Or, did you download the Media Creation Tool for the upgrade.

I have upgraded two Core 2 Duo laptops and one desktop with an older Prescott P4 single core CPU.

The Optiplex 745 should upgrade to Windows 10 without issue.  You might have a video card that is not supported or some other piece of add-on hardware that 10 is having an issue with.  I've an Optiplex 745 sitting in another room that is running 10 just fine.   

However, 10 is  NOT an upgrade from 7 in my opinion.  10 is chock full of spyware that you have to agree submission to in order to use the OS.  I won't use 10 for anything more than testing.

If you still wish to try, use MS' media creation tool to download which ever version of 10 you want to try and create an ISO.   Use the iso to burn install media or extract the iso to a usb drive and install using one of those two methods.  Being off line while doing so might be beneficial.   

Normally, you have to do an update to an existing, activated Windows 7 or 8 to use the free upgrade, but there is a way to do "clean" install and still get activation.   I won't outline that method here but it involves copying ONE file from the Win10 installation files and running it on your 7/8 install to get a necessary file output.  That output is an XML file that is then copied to the installed Win10.

Dell is making a very large mistake by supporting Windows 10, IMHO.   We shall see.

Ensure you have the latest (MBR) BIOS version installed 2.6.6:

http://downloads.dell.com/FOLDER00224415M/1/HEIDEN_2.6.6.exe

Also if you are using an ATI/AMD graphics card particularly the X1300 remove it and use the onboard Intel video.

Try the upgrade to Windows 10 using the Windows 10 Media Creation Tool:

http://dellwindowsreinstallationguide.com/upgrading-to-windows-10/