From here. Works on these knock-off mini-dongles from DealExtreme. If you have this exact dongle, you can skip all this and just use my hacked INF.

• Insert crappy dongle.
• Skip the drivers.
• Open the device in device manager.
• In the Details tab, select "Hardware ids", and select the entry that has the Vid and Pid only – the one that looks most like "USB\Vid_1131&Pid_1004". Press Ctrl-C to copy this entry to clipboard. Save it for later.
• Copy the file "%windir%\inf\bth.inf" to another directory.
• Open the copy in an editor (e.g. notepad), and find the "[Cambridge.NT.5.1]" section.
• Add the line "My wacky dongle=BthUsb, USB\Vid_1131&Pid_1004", except replace the bolded part with the string you copied earlier. You can change the name to whatever you like. Save the file.
• In the device properties, click "Update driver" or "Install driver".
• Choose advanced, and specify the path to the "bth.inf" file we just edited.
• Your dongle should be recognized as "My wacky dongle" or whatever you named it, and work with the standard XP bluetooth stack.