new dell 8300 dimension computer with windows XP professional. Black shadow on the AOl screen.

Since purchasing a brand new dell 8300  dimension computer with windows XP professional. I  can see a black shadow on the AOl screen.

This shadow runs underneath the menu bar and above the task bar. On at least three occassons  rectangular shaped black areas have materialised and as I have minimised the windows of the favourites, buddies and welcome box the black shadow appears behind these boxes.

I have not noted this in any other software  program so far.

Please advise thanks