I also have XP, use Firefox and have an xul.dll in that location. Your dll file may be corrupted. Firefox 3 had two updates last week, the newest one being 3.0.3. I would suggest uninstalling Firefox, opening IE and downloading the newest Firefox version from:
and installing that.
While on my computer tonight....I went to open browser, which is Mozilla...and a download started....not sure if I cancelled it or what..but now I get this error.
The Application or DLL C:\programsfiles/mozillaFirefox/xul.dll is not a valid windows image. Please check this against your installation diskette
I am running Windows XP and it is a Dell