Click on the link below and copy and paste all the diagnostics here (in a code box)
steam://support/?Issues=*
Not to be a jerk but you ignored my post...
Ah, forgot to mention it gives me a fatal error. Sorry about that Rabid.
CsComm Session Mar-17-2009 22:19:09.716 [1212] ReconnectThread (1212) Starting
CsComm Session Mar-17-2009 22:19:19.834 [1212] ReconnectThread (1212) Told to exit
perhaps download ccleaner and run a registry scanner to clean it up.
-Blue
Alright, open the following directories,
Crogram FilesSteamSteamapps<yourusername>Team Fortress 2
Crogram FilesSteamSteamapps<yourusername>Counter Strike Source
Crogram FilesSteamSteamapps<yourusername>Insurgency
And delete the HL2.exe.
You may also want to clean the registry as blue stealth said.
If this doesn't work, preform a full virus scan with your system, as it could be corrupted.
Finally, if this doesn't work then its time to back up your files and reinstall windows.
Tried both of those, nothing.
Dont format yet...can you explain your problem a bit more? Can you open steam, just not open a game, correct?
Okay open steam, right click one of the games, go to properties, local files tab, Delete Ingame content, and verify your cache. Then try the game tell me what happens this is just a normal procedure. Do you still have the games with you etc... perhaps uninstall and reinstall the games...this is what i had to get TF2 to work, reinstalled 3 times.
Tough problem. very tough. whatever is happening is happening in between Steam and hl2.exe somewhere. try this; add a non-steam game, then try to run it by launching through steam. then try a steam game that doesn't run through hl2.exe. see if those work okay.
also, make sure the permissions for hl2.exe are standard, try setting it to run as administrator if necessary.
if you can remember, did anything happen to your computer in between the last time you could play and the first time you couldn't play?
EDIT: A thought. any drivers you may have recently updated, try rolling them back. particularly video drivers. or if any of your drivers are out of date, update them.