Hello and Welcome to TSF,
Backup your important data immediately if you have any on the harddrive.
Check your ribbon connections
Possibly a bad ribbon could be too
Run a diagnostics on the harddrive using a utility:
http://www.tacktech.com/display.cfm?ttid=287
Find the one for your harddrive here and burn the image as an iso to a cd disc.
Note: personally my good guess is that it is the harddrive failing.