Well, strictly speaking, the OEM version isn't a kosher license, as it's only supposed to be supplied with a new PC. An official full copy/license should cost in the region of 225UKP. If you have an older O/S, you should be able to pick up an upgrade version for less than 100UKP.
Is it the full product you need, or do you have a license but have lost the CD? If you have the serial number, you can use that with any CD, so you could get a dodgy copy from a computer fair for a few quid and use that disc.
Heath