I experienced exactly the same problem. This is so horrible. I hate this. I just bought a new laptop 2 days ago with 4 GB RAM.
I am just browsing (only 10 to 20 tabs) and I get a message saying Windows lacks memory.
Task Manager says only 70% is used (70% ! With only Firefox opened taking 200MB of RAM ???)
Well anyway, if I ever happen to launch another program, I will surely get logged out. This is so **** me off...
It already happened 3 times today and twice yesterday.
Is it a gimmick to make me buy more RAM ?!
Well, I'm switching back to Windows 7 and the marvelous Windows XP, which I continue to use for some old piece of software that doesn't work anymore on Win7 nor Win8.