Help waterprofing desktop, or finding replacement.

Hey all, 

I've got a Bridgeport BTC-1 milling machine, controlled by an old Dell Optiplex 755 and a Mesa 5125 card. The 5125 card connects to a Mesa 7177 card with a parallel port cable. It is in a barn without AC, and it seems the Dell is the weakpoint. It often breaks, probably due to the heat and humidity. I can get new ones for about $80 on Ebay, but would prefer to have a more permanent solution.

Can anyone recommend a good computer for this application that wont' break the bank?

Thanks, 
-Hunter Long

I was wondering if a conformal coating would help, and found this thread, which may have some relevant ideas:
electronics.stackexchange.com/questions/...nd-will-this-prevent