See
this page on Intel's site. Win 7 is Microsoft's apology for the turd they called Vista. Unfortunately, there are no Win 7 or Vista drivers for your chipset, either so you can't get there from here with the 845 graphics chipset.
A machine based on that chipset, in fact almost ANY machine, will run faster and smoother on XP, which is still a good running OS. I suggest getting a copy of XP.
The only limit you'll hit with XP is that Microsoft is soon discontinuing support for all versions older than SP3 if they haven't already done so. The answer to that is to search Microsoft's site, the forums and Google for information about slipstreaming SP3 into any older version of Windows XP.
This is a perfectly legal use of an otherwise legal copy of Windows XP. The actual update files files are available on Microsoft's site. Instructions on how to integrate them into a working installation disk are easily found.
Hope that helps.