Problem with GDM on Ubuntu 8.10

[Samuel Rohde]

When I turn my computer on and it comes to the GDM screen, my screen goes blank and it says invalid format. I can still log in because I don't really need to see it, I just type my user name and password. Once I'm logged in everything displays normally. I switched screens to see if it had something to do with the size of the screen, and I was then able to see the GDM. The screen that I can't see it with is a 32" wide screen TV/monitor. How can I fix the GDM?