Are there any parts your friend can bring over from an old PC? Just to get a bare minimum of case, PSU, motherboard with integrated GPU, CPU, memory, hard drive and optical drive is going to cost about £235 minimum (the above spec includes a third party cooler which is probably overkill). The Atom-based complete systems linked by Ast3r are a similar price. Add to any of the above options the cost of Windows and you're over the £300 mark already, so without bringing something over from an older PC the price range you've specified is a bit of a tall order.