On Scan's "Today Only" page today (
http://www.scan.co.uk/TodayOnly), first on the list is:
Intel Core i3 540 Dual Core Combo
* Coolermaster Elite 330 Black/Silver Midi Tower Case w/o PSU
* 650W EZCool Silent, SLI/CrossFire, 120mm Silent Fan, ATX12V, AMD/P4 Ready
* Intel Core i3 540 Dual Core Clarkdale 32nm, 3.06GHz, 4MB Cache, 73W, Integrated GPU @ 733MHz, Retail
* Asus P7H55-M/USB3, Intel H55, S 1156, PCI-E 2.0 (x16), DDR3 2200(OC), SATA 6Gb/s, USB 3.0, mATX
* 1TB Samsung HD103SJ Spinpoint F3, SATA 3Gb/s, 7200rpm, 32MB Cache, 8.9 ms, NCQ, OEM
* 4GB (2x2GB) Corsair XMS3 Classic DDR3 PC3-12800 (1600), Non-ECC Unbuffered, CAS 9-9-9-24, 1.5V
£269.99 inc. VAT
So that's a case, power supply, processor, motherboard, hard disk drive, and memory. All you need to add is a PCI Express graphics card and copy of Windows 7 and you've got yourself a damned nice PC! The deal lasts until 13:00 tomorrow, when they refresh their bargains page.
Incidentally, Scan also offers OEM versions of Win7 at good pricing - I picked up Win7 Professional 64-bit OEM for £108 (it's not on the Today Only page). If you don't need the Pro version features, Home Premium is there for £72 including VAT (also not on the Today Only page - you need to go into their catalog to find it).