So, just to clarify...
If I purchased an
OEM version of Windows XP SP1 or SP2 (I forget which) for my PC build 3 years back, I could theoretically use an "upgrade" version of Windows 7 to make the jump.
NOTE: I probably won't actually do this, as I have read articles stating that older computers (especially those "designed for XP") will not meet the recommended system requirements for 7, and as such, will run too slow to make worthwhile.
EDIT: Also, I know about the Windows 7 Upgrade Advisor app
[link], which I'll probably run later today for a better idea of my PC's suitability for an OS upgrade.