can you get the exact message concerning the missing file?

Can you start up in safe mode? reboot and immediately start toggleing the F8 key till the start up page comes up asking to boot into various configurations. first try to reboot into the "last good known configuration" if that fails return to that page and boot into safe mode....when in safe mode...navigate...start>programs>accessories> system tools> system restore...try to restore to a prior date from the calendar(the bold dates are the restore points that are available)

Post back with the details.