I just started running into this about a month ago. Basically when I boot my machine everything runs fine, but if I walk away and come back a few hours later I can no longer start new processes. This includes:

* FireFox
* IE
* Visual Studio
* Task Manager
* Rebooting or shutting down the machine

I can still move around in open programs, but starting anything new does not work. Does anyone have any ideas as to why this would happen? I've done a virus scan and nothing comes up. 

Any suggestions appreciated.