dual boot 2000 on c: / xp on d:

Dear Geeks,
Win 2000 Pro SP4 on c: working fine...

Win XP SP2 installed on d:, worked for awhile, then system.dat crapped for
whatever reason ?, copied an old one, boot from d: ends up with a bsod 50...

Is there a reg cleaner that can fix reg on d: while booting from c: ?

Can XP be re-installed on d: without having to re-install the programs (a lot
of them) already installed on d: ?

How to protect c: if I decide to re-install XP on d: ? I cannot loose c: ...

Cheers and ... have a great geek day,