Which version of Windows are you running? For XP and Vista the last known configuration that worked or simply booting up in safe mode to disable different items in the msconfig utility until isolating the offending program are the two main options used.
In XP you can choose the safe mode /command prompt only option found in the Windows F8 startup menu to manually startup the system restore feature as another method of restoration there. Simply follow the basic instructions seen at How to start the System Restore tool at a command prompt in Windows XP
As the last resort prior to a full reinstallation of Windows and all programs if XP is the version being used there the repair install method generally works quite well. Review the article on this seen at How to Perform a Windows XP Repair Install