|You have to boot of the Windows XP CD. You will get two repair options. The first one will take you into the recovery console that will give you a prompt. You can perform various repair options from there.|
Ignore this one and you will get a second repair option. This will do an repair install, overwriting the current installation but keeping all your settings and configuration.
There is no repair option in Windows itself, but there is a System Recovery option that will put your system back to a previous date when everything was running correctly.
Which one you use depends on exactly what the problem is.