Acer Celeron 2.66 GHz desktop + 17" TFT monitor - £299 or less delivered
Go through Quidco for 3.5% cashback.
Specifications: Intel Celeron 2.8GHZ processor, 80GB HD, 512MB RAM, CD/DVD-RW,Windows XP home, 6 USB2.0 slots, 9 Media Card slots, 10/100 wired networking built-in, Built-in 56k modem. AGP x8 Graphics card slot.
EXPREX 17" TFT monitor (8ms response time), Keyboard and mouse.
Thanks to RIKER71 for the specs update :)