If you have one near to you, go to an independent computer shop that builds PCs, these will usually also sell components.
Tell them what you want the PC for and get them to build one that will fulfil that function.
This may work out more expensive than high street vendors but it will be worth it.
As an example, if you want to play the latest games, then you will need a fast processor, plenty of ram and a good graphics card, and a large hard drive, this setup would also be suitable for graphics applications, DTP, scanning, and video editing.
If you only use business apps, e.g word processing, spreadsheets, etc, then the processor and graphics card are not so important.
There are two paths to choose, Intel or AMD processors, both use different motherboards, memory and have different power requirements, so choose carefully, as once you have chosen one above the other you are stuck with it.
AMD have the advantage in pricing, Duron processors are good for games, and can be picked up quite cheaply, a Duron 1.1Ghz can be bought for about �40 and an Athlon 1.4Ghz for about �80, whereas the corresponding Intel chips are about �20 - �40 more expensive, depending on where you shop. Graphics cards can cost from �15 to �300 depending on what you want.
If that is not for you then choose one of their prebuilt systems and add the components you want, e.g my local supplier has a system for �795 + vat which would be a good games machine with the addition of a good sound card to replace the on board sound and some surround sound speakers.