CS:S won't start; cache needs repair

[Elu_Thingol]

I've recently preordered the Bronze HL2 package, and at first it worked, but it doesn't anymore. When I start the game, it says Error 116: Cache needs repair, and exits. How do I fix it?
My specs: Pentium 4 2.8 w/ HT, ATI Radeon 9800 pro 256Mb, 1 gb PC3200 DDRAM, 120 GB HD.
Thanks for your help.