Have you checked for updated drivers from nvidia? They might have updated drivers if Dell does not.

I called the support, and they try to fix it. I re-installed nvidia, quickset, biosupdate and updated all tools by dell. The support couldn't fix it. And at final advice I probably shall re-install windows, but at this time i don't want to.
Hopefully the next nvidia driver will fix it. For this time I selected, turn into Standby after closing. Then the problem doesn't appear!
But I don't want Standby, cause some Programs which have t be run in the background, need to be connected!

btw. the newest Nvidia drivers from their page, aren't combatible with the fx2500...

Thats too bad, it must be because its a portable system. It must be ATI that is supplying portable drivers direct. It would be great if nvidia would start doing it since it could be a long time before Dell updates them.