Out of date video drivers and OS packages are known to cause problems when
using fullscreen mode, so check for updates. You should have at least Windows
98, DirectX 6, and recent video drivers (or MacOS 9 or DrawSprockets 1.7.2
if you use MacOS 8). Also, try deleting your "WhiteCap Prefs" file in the case
that it's become corrupted (there's an endless number of ugly things that
can happen from a damaged prefs file).
It's also possible your prefs file is corrupted or contains a setting causing serious problems. If you've been recently experimenting with your prefs file, this is most likely the cause of your problems. See the following question to make a new prefs file.