I use both a CompUSA branded and a D-Link bluetooth USB dongle. You should be able to get a BT dongle for about $20 (usually after rebate).

Generally pretty easy to install, but don't try and mix and match once installed as the serial number will screw up the drivers. Oh, and you generally don't want to use the BT drivers installed by Microsoft in XP but the Widicomm drivers that ship with your dongle.