Do you run Windows Aero with your fireworks? If you receive a message that the color scheme has been changed to Windows Vista Basic, one of the following might have happened: a program that you're running is incompatible with the Windows Aero color scheme, or your computer does not have enough memory to run all of the programs that you have open and also run the Windows Aero color scheme.
Try closing some windows or changing your color scheme to Windows Vista Basic, use some tools, such as freeram, frees wasted memory back to the pool of available resources. If you changed your screen resolution, video card, or monitor setup, your computer might no longer meet the minimum recommendations for running Windows Aero.
I had some problems too with Fireworks and Windows XP SP2 when i added a custom theme.
Anyway, just formatting the pc the problem disapeared.