depends why you want to convert. is it for utility purposes in the city? is it to improve your pedal technique?

if it's for barspinz, don't bother. however, i do think some of the other people are being harsh. keep in mind, converting a bike without any components/tools lying around may require about $300, less if you keep a bunch of the old components and rotafix the rear cog in lieu of getting a hub or wheel made for fixed gear. if you're going the deep-V route, that'll up the price, and will pretty much just mean that you should just buy an off-the shelf bike for $500 anyway. then, you'll have a fixed gear and a nice older bike.

EDIT: WTF? it looks like it's already a singlespeed in the top right picture?