If you've ever been to a chain store, you've probably seen bettas kept in tiny (~6 ounce cups), so yes, a betta can survive in your jug. This is because bettas possess an organ that allows them to breathe oxygen from the air to supplement their gills.
However, I would still pick up a new tank. A new tank won't be very expensive, and in a 1/2 gallon a betta won't have much room to move around. Again, if you've seen bettas kept in the tiny cups you'll see that they don't look very good... bettas need space to encourage them to get exercise and show off their finnage.
A 5 gallon tank would be ideal, but that might be a bit big for an office tank. A 2 gallon feels like a good compromise, not too big, but still roomy enough for a betta. Throw in a lid (bettas can and do jump), and maybe some gravel and a plant (plastic is fine, though an anubias/crypto/java fern would be great), and you have an excellent habitat that shouldn't cost you much more then lunch. Another bonus to a larger tank is that your water changes can be less frequent.
For more info Betta Talk is a good site on Bettas run by a professional breeder. A link to the section on housing...
http://www.bettatalk.com/housing.htm
