Go here and look at the options that you have as you failed to mention what OS you have... although these repl to other OS than xp it also applies to xp and xp pro as they are based on the NT file system...
The most common repair would be to use the original microsoft cd and boot to it and choose a repair option (by pass the first repair option by choosing to install xp...this will lead to another repair option after it detects the xp set up ..that is the repair option that you will want to use)
you will not loose any personal files but you will have to reinstall the SP1 orSP2 service packs again..If you choose sp2 you need not to install sp1.
