PC boots up in Win98 instead of XP

Hi

A few weeks ago I installed XP on my PC which has two HD one 80Gb (dirve D) and one 7Gb (dirve C, which is older). The C drive is mainly used for Windows and I try to install all other programs on the D drive. A few days ago my computer started trying to boot up with Win98 (the previous op system on the C drive) and failing. So now I have the PC boot up on one of my CD drives using the XP disk. I have installed XP again (more than once) but no change. How can I fix this, will I have to format the C drive or what?
HELP!!!!