GARDENER,
I am going to take a wild guess here. You are using the Hard Drive from your old computer and you put it into this one (using the old WinXP OS that was on your old computer).
WinXP does not like that many changes to the hardware. You need to do a Repair with the WinXP CD. If that does not work, you can do a fresh install of WinXP on top of itself. (That way you do not lose any of the Data on the Hard Drive, other than the Data in the Windows Directory.
Take Care,
Richard