|mmm before you go about a repair installation (as suggested by "ace...") might be wise/useful to first secure data - off the system entirely.|
A repair installation normally doesn't affect data; doesn't wipe it etc... But better to safer than sorrier?
Boot with a Knoppix or Ubuntu disk and then locate data and copy to optical media. Verify the copies are truly accessible on at least one other system too.
The set about posssible repair etc...
Each has a freebie iso image which download to a hard-drive. Then burn to dvd; use that dvd to boot etc...