Microphone to USB Lead - £19.99 instore (or £22.98 delivered) - Maplin

As a MAJOR additional benefit, this is compatible with Guitar Hero WT and Rock Band 1/2 (at least on the PS3 and 360), meaning if you have a half-decent XLR microphone already, you can use this cable with it instead of the shoddy mic which comes with the band kit.

Here's the bumpf from the Maplin website:

* Record from a microphone to PC with ease
* 16-bit 44kHz mono recording
* 5 metre lead - USB on one end, XLR plug on the other
* Suitable for other audio devices that accept XLR plug
* ''Plug ''n'' Play'' connection
* Powered by USB no power supply required
* Power on LED indicator light
* PC and MAC compatible

The Microphone to USB Lead makes it easy to connect most XLR microphones or other XLR audio sources to your PC. The lead records in studio-quality 16-bit at 44.1 kHz using the internal analogue-to-digital conversion process. The Microphone to USB Lead is ''Plug ''n'' Play'' (no drivers required) and is compatible with PC and MAC. Maplin recommends use with good quality sound recording software, for instance Audacity.