Try starting up in Safe Mode. To do so.....
Power on your Thinkpad and immediately start tapping the F8 key along the top row of your keyboard. This will bring up the Windows Startup Menu (I am assuming that your are running Windows!)
Select Safe Mode from the menu using your arrow keys and then press the Enter key.
If this allows you to startup in a basic Windows mode, I would then recommend you perform a system restore from Start->All Programs->Accessories->System Tools and choose a restore date a day or so previous to your problem.