Matt_2006 - That big circle is on the standard Welcome Screen, but is more evident in this screenshot provided because it is saved using only 256 colours by the look of it.
The user checked and his colour quality is set at highest.
Does this problem still happen when you Log Off or Switch Users, or just when you start Windows?
All the time
Have you noticed any other Windows logos etc missing?
No
Possibly a video card problem (get latest drivers) or font problem maybe? I'd say repairing Windows would help, but unless there are other problems, it wouldn't really be worth it in my opinion.