This has happened to me a few times, with the Source engine playing Counter Striker : Source.
I found it was related to my keyboard and mouse drivers, and I think Fraps interfered it was a previous version.
Can you post a DXDIAG log?
Start / Run / type DXDIAG , save all information, copy and paste in here.
And a HIJACKTHIS log?
scan and save log, copy and paste in here.