Your master boot record is corrupted or your BIOS isn't booting from the correct HDD. My guess is its probably the first one, but double check your boot order in the BIOS. There's ways to repair the master boot record if you made a copy/image of it. I'm not quite sure of the exact process (its been a while); you should be able to do it from the repair console. If you don't feel like going through all that, you can boot from your XP CD and repair your current installation by going through the install steps normally and selecting the partition where your OS resides. It'll then give you an option to reformat/reinstall or repair the current installation. You'll need to reapply any SP and Windows Updates afterwards, but all your programs and files should remain intact.
Chiz