I presume you're meaning the Upgrade Assistant from MS? If you're running WinXP you need to use the checker for Win8 not Win8.1, at least that's what MS says on the web page:
Upgrade Assistant: FAQ - Microsoft Windows Help
Might explain the inability to run. I'd agree with Mike that from those specs your PC should be able to run Win7 or Win8.
There's no point in downloading Win7 from the suggested site if you don't have a suitable licence key.