windows xp wont start

My girlfriends computer wont start i have 2 different installs of windows xp on it neither will start up . i can pick safe mode, last known ,start normal no matter how i try to start it it just restarts in a endless loop on both os's .

I have installed another hardrive and the computer boots right up no problems.

When i slave the bad drive i can view everything and I have tried running windows repair, erd commander to try system restore and nothing changes it from its endless looping.

All hardrive tests i ran says the hardrives fine.

Do i have any chance of repairing this drive without a reformat and even if i do reformat does it sound like the drive is damaged somehow?

I have moved everything to a external but my girlfriend refuses to reinstall and lose her stuff. Its been 2 weeks i really want to get it over with please help.

Thanks Dave