Just a quick "guide" really but this may be of use to someone.
Last year, friends of mine purchased a Windows XP computer which, to cut a very long story short, was replaced with a new Vista PC last month. The Vista, however, unlike the XP ran incredibly slow, I tried to access a page on the computer and it took 12 minutes for the page to come up on 8MB Broadband even though my laptop next to it running on dial-up brought the page up immediately.
After much elimination of possible causes we discovered that the problem was caused by the anti-virus software supplied with the computer - McAfee. Uninstalling the software IMMEDIATELY made the computer run at its correct speed.
Bizarrely, the McAfee software was supplied with the PC and it would therefore seem compatible. It's not. Although in the future McAfee will undoubtedly work with Vista, if your Vista PC is running slow with the software it is recommended to use an alternative. AVG is one of the best and most reliable anti-virus software programs and can be obtained free (cannot post non-eBay links but type AVG into Google). Install AVG prior to uninstalling McAfee to ensure you are protected at all times and then go to Start > Control Panel > Add Or Remove Programs > McAfee to uninstall the anti-virus culprit.
A BT engineer whom I know personally told me that this is causing a big problem with Vista users yet is still little-known. If you have a Vista PC and McAfee anti-virus software and your computer is slower than it should be this may well be the cause; within seconds of uninstalling it the computer I was on ran at the speed it was meant to.