�80 is hardly a bargain Indie, Ebuyer sells them for around �50.Most of Maplins pooter gear is well over-priced.
The problem could possibly be cured by a re-install of Windows but is more likely to be a hardware fault such as a dry joint on a contact of a module which when it has heated up after 20 minutes makes a proper contact.
Beegeebees suggestion is a good course to follow.The make of motherboard might be shown up on the first POST screen. When you have ascertained which BIOS your PC has, look up the code on this page
http://www.computerhope.com/beep.htm