You can't expect an OS installed on one PC to boot up on a different one as the motherboards are probably different.
If you're using XP, you could try a repair :-
Boot from the WinXP CD (in the BIOS boot sequence put the CD drive ahead of the HDD). When the first message comes up, choose “To set up WinXP” not “R for recovery console”. Windows will eventually recognise that XP is installed and ask what you want to do, press R to repair and follow the instructions except after rebooting don’t reply to the message “Press any key to boot from CD”, just wait.
Your personal data and non-Windows programs (with a few exceptions) will not be affected. You will have to install all Windows updates that are not on the CD, e.g. SP2 plus about 40 fixes.