Help: XP crashes after being idle

When I leave my  8200 Dell computer on and idle for a few hours and come back and click on an icon I get insifficent system resources to run. Nothing works. If I reboot all is OK. Whne I do reboot the windows shutting down picture is very tiny when it normally is full screen.

The only power setting I use is to shut the monitor nothing else. Ran scannow and chkdsk. Also did Dell system check with no errors found in hardware. Checked for spyware and adware. Hard drive is only 30 % full. Temp files empty. Bare minimum set to run  on start up. Ran task manager all the time and see nothing unusual in processes or apps when the problem occurs. Dell spyware is disabled.

System is 2 gig 512 RAM and Windows XP home.

I can leave it on for 2 days click on an icon and all works, Then come back 2-5 hours later click an icon and Desktop insufficent resources comes up.

Any ideas?  Thanks

fjx1@aol.com