but the question about how old is you pc is valid because here is the thing.
One is with windows 7 on old system with limited memory and hard disk space you might have a real slug.. say 2 gig mem and 250 gig drive minimum but more far better
And more important you might be able to find a low end new or refurbish pc with better hardware then your old xp box and WITH WINDOW 7 License on it for the same cost as just buying window 7 outright
Try microcenter..there on line but if a store near you the better..look at lease returned referbs..and store return. And look at bundle deals...
then make you old pc a Linux box sell it if you want...even sell you xp license..get another 50 60 bucks out of both
Just fyi..I just check at microcenter on line
window 7 home oem by itself is 99 bucks
a lease return box with window 7 License on it is 89 bucks with 1 gig mem..
99 bucks for a lenovo with 7 and 2 gig of mem and 160 gig drive and core duo