Your limited to 3 hardware upgrades then XP tells you you need to re-register or it stops working in a preset amount of time

Here's a link to some posts about that

http://www.netsmog.com/cgi-bin/yabb/...139934&start=0

And here's the world's easiest crack.  This was proven on a beta but may still work

http://www.theregister.co.uk/content/4/20433.html

According to this note on the register just copy that file back from a backup copy and as long as your memory didn't chage your fine.  You rally need to read this though it goes into great detail as to what will trigger a re-register notification and how long you'll have.