That bike is an excellent candidate for singlespeed conversion.
If you just want to convert this to a singlespeed, but not fixed-gear, then it's a very simple matter:
1. Remove the rear freewheel cluster using a freewheel remover (and probably a bench vise) -- Your bike does not have a cassette, so the freewheel screws on to the hub.
2. Screw on a new 3/32" bmx freewheel -- maybe a 19t. 52x19 is a really nice gear.
3. Remove the shifters, derailleurs, and the inner chainring.
4. Shorthen your chain.
5. Tighten the crap out of the rear quick release skewer.
6. Ride your bike.
Total cost should be under $20 (assuming you already have the required tools)
Your chainline will be way off, but for a winter beater, who really cares? If you really love singlespeeding (and you probably will) then you can always spring for a new wheelset -- fixed / free, or whatever, but you'll probably need a new crankset to get a better chainline.