There's some discussion about this on the Dell forums.(assuming that this is the same issue) http://en.community.dell.com/forums/p/19242368/19372866.aspx
Do you have the video card that it mentions?
Couple of things that you could try from the suggestions in the link:
Blow out the fans with a can of compressed air.
Upgrade to the latest bios for your system.
If that doesn't help, since your system is likely still under warranty, call Dell. Most have a 1 year parts warranty.