I am using the assumption you have master/slave jumpers set correctly and are not using cable select. I also assume your bios is reporting the correct size drive. If you have not checked this do so. If you have check again to be sure.
If you install sp2 for XP then I hope you followed the directions by the letter. It can reak havock on your system if not.
If all looks good up to this point then I would try using a partition manager. There is a pretty good one here,
http://www.ranish.com/part/ , called Partition Manager. It has been in beta a long time but DON'T let that steer you away. It does things Partition Magic can't.
This should give you a good indication if your partitions have been corrupted. If so this app has some utilities to corect them as well.
Sorry for the lack of help but without knowing the above first it is hard to track down the problem. Even experts jumper wrong or set the bios to user with wrong settings. Then spend hours trying to figure it out until they realize the obvious is staring them in the face

take it from me, I've done it.
Good luck!