You could try booting into safe mode. As it starts to boot, keep tapping either the 'F8' or 'F5'key. If it lets you in firstly try a 'System Restore'
If that doesn't sort it you can run checkdisk.
Click 'My Computer'
Right click 'Local Disk (C:)'
Under the heading 'Error Checking'
Click 'Check Now'
click the box...
'Automatically fix file system errors'
Click start. It will say it can't until you reboot.
Say OK and reboot.