It's odd that Episode 1 worked before (i assume you implied that playing through them), but now it doesn't. It's possible that some security software updated its signatures and now blocks it.
Please try running the game during a clean boot:
Booting clean in Windows Vista / 7:
1. Click Start --> Type msconfig --> Press Enter
2. On the General tab, choose Selective Startup
3. Uncheck Load Startup Items
4. Select the Services tab
5. Check Hide all Microsoft services
6. Click Disable all
7. If running the game on Steam, re-check the "Steam Client Service"
8. Click on OK
9. Click Restart.
10. After reboot, run the game to see if it works.
After performing the necessary steps, restore your system by doing the following:
1. Click Start --> Type msconfig --> Press Enter
2. On the General tab, choose Normal Startup
3. Click Ok
4. Click Yes, when asked to restart your computer
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