Personally, I wouldn't recommend depriving yourself of the fun experience of beating the game the way it was designed to be, but if you insist you could do a couple of things to make Desaan a wimp.
The most straight forward to what you're asking is to edit his npc file. You can find a tutorial about how to do this here (note might vary slighty as this is for JKA not Outcast, but its basically the same format):
Note, that you probably want to make a seperate pk3 to store the modified Desaan .npc in, so as to not overwrite the original gamefiles permanently. The easiest way to make him a scrub is change his health value to 1hp, but have fun trying out all the new stuff. You could change his class, or lower his accuracy, aggression, etc. Keep in mind doing this will probably mess up your save file, so don't mod until you have a backup save.
Which version of internet explorer are you using? Have you tried using using internet explorer with add-ons disabled? (Windows Key + R” then Type “iexplore –extoff) Have you reset to factory settings? (tools > internet options > advanced > Reset...)