This is certainly not the fastest computer money can buy and it hass only six month warranty but this bundle is the cheapest in UK at the time of writing.
This Acer Aspire T180 packs a Sempron 3200+ processor which runs at 2GHz, plus a 512MB DDR2 memory and a 80GB hard disk drive.
There's a DVD writer, an integrated Geforce 6100 GPU with 128MB shared graphics (ed: reduce that to a mininum and grab 4GB DDR2 memory).
Other features include High definition 7.1 channel audio, GigaBit LAN, 56K modem & card reader and most importantly Windows XP media center rather than Windows Vista.
You will also get a PS2 mouse and a keyboard as well as a 17-inch LCD monitor, all for the £176.24 from Morgancomputers.
The T180 Chassis should give you enough expansion to last for a while and to keep your internal components cool.