Cheapest Dell PC currently going for around £250 to your door. You could get components to your door to build something similar for £200. Spend £250 on components and you can build something that would have been very high spec only a few years ago. Depends what you need the PC for, you can spend £500 on a graphics card if you want to, or 'nothing' as it is included on the board.
Also remember you can get a copy of XP from the component suppliers for around £50 and it is legitimate. Building PCs is dead easy, biggest gotcha is not pushing the bits in properly, really depends how brave you feel...