Unable to validate, what's this?
Well, little is known by me.
But I know that it sometimes happens because of the MarsWars.ini or MarsWarsUser.ini
Make a backup of both (altough they aren't that important), and delete them from MarsWars/System.
Launch UT2004 again. The inis will be re-created and set everything to default.
General protection fault!
Have you patched UT2004 to the latest version 3369?
The patch can be found here under "Downloads" -> "Patches"