Try a repair-install. Boot from the Windows CD/DVD. Select repair existing installation. Follow the wizard. You will not lose any data, but the registry and dll's will be restored to default. Also, some third-party drivers might need to be reinstalled, no biggie, you can get them from the website or the Driver CD that came with the system.
Alternatively, if that fails, you can do a System Restore using the CDs that came with the system; you will lose the data with that option, the laptop will be restored to factory clean state.