'The memory could not be 'read'' at startup.

[JMS]

So today I bought Episode One and HL Source, but when I start up either of them - or any other Source game for that matter, even though they worked perfectly a couple of weeks ago - it crashes. It shows the Valve splash screen and the blurred out background picture for the menu, but then right before showing the menu, it crashes and the error message says: 'The Instruction at [address] referenced memory at [address]. The memory could not be 'read''.

It's strange because no other program or game on my computer does this, and source games didn't use to do it either.
Any suggestions?