Did you recently make any changes to Windows 7? If so, see if you can boot into Safe Mode
to System Restore to before the changes were made.
If not, try from Safe Mode running a clean boot: Clean Boot Procedure
If you can't boot to Safe Mode, boot your Windows 7 DVD Repair Console or System Repair Disc - Create
to run Startup Repair repeatedly, which will automate System Restore, SFC and DIsk Check as well as many other tests and fixes.