If you "upgrade" your computer and XP will no longer activate, you need to phone the microsoft product activation helpline. Windows generates a huge code which you either tell the operator, or type into your touch tone phone, then another huge number is read back, which you enter into Windows XP and hey presto it is now activated on the new or upgraded computer. Simple and painless operation.
Swapping the hard disk into a new computer probably won't work, especially if the MB chipsets are different....
Cheers