Half-Life Problems...

Hey, Im new here. I've asked this question on the SteamPowered forums but they havent been able to help me, and then i remembered this site...
My Problem: I borght the game last week (half life 2 - game of the year edition), installed it and made a steam account etc... But when i click "lauch game" all it says is "preparing to launch." Then the cancel button blanks out and goes back to my normal screen, with the game not loading
Ive set my firewall to allow half-life and counter strike as exeptions so that cant be the problem...

My Pc specs are -
CPU Intel Pentium 4 3.0 GHz HT
Motherboard MSI MS-7060
Memory 512 MB DDR RAM - PC3200
Hard Drive 120 GB
Video Card SiS 661FX
Sound Card Realtek AC'97 audio
Modem Creatix V.9x DSP Data Fax Modem
Network Card SiS 900 integrated fast ethernet

So the game should work fine but it doesnt...

If i could get any help on this matter, i would really appreciate it! Thanks!

 

did you restart your computer and reinstall half life 2?