Try booting ur system in the last known good configuration mode . Press F8 at startup to do this .
If it boots , install the recovery console from ur windows XP cdrom and try repairing it then .
If the system doesnt boot in LKGC mode, boot with ur windows xp cd (Make it as the 1st boot device ) and try doing the repair installation.
I suggest you not to go for a "format ".