You may need a BIOS Update...
Click Here to Go to the Dell Site...Expand the BIOS section and download the BIOS update, full instructions are given, but its a simple file download and run
Also check the Video drivers are the latest available, check the version by:-
Start > Type "device manager" and Enter > Collapse "Display Adapters" > ATI Mobility Radeon HD 4500 Series > Drivers Tab
You should have version 8.6....
If not use the link I have above and then expand the video section...
If that is inconclusive then check Windows Update and if not then try contacting Dell regarding the issue...