Would you consider building your own?
Are you going to be playing PC games? If yes, which ones?
Do you need a monitor (TFT LCD? What size?), keyboard+mouse or other peripherals?
Do you have to store plenty of music, pics & videos?
Do you have any experience with Windows Vista? Does WinXP do all you want it to do? People either like or dislike Vista for many reasons.
Do you have a budget, & if yes how much?
You can get a basic readymade PC for £170-180 (or less in some places) - e.g. a Dell Vostro 200 click here. All you need is a monitor. May be a bit short on hard disk space, but these days you can pick up a 500Gb internal one for ~£40 & external for ~£50.
Just over £500 will get you something substantially better with a quality 22" monitor & 500Gb hard drive included click here. G