ok cheapo sempron cpu about 25/30 quid. Mobo with onboard gfx another 30/40 and 20 quids worth of ddr2. You hdd optical case and psu(if its atx)
http://www.ebuyer.com/product/134751 cpu
http://www.ebuyer.com/product/126138 mobo
http://www.ebuyer.com/product/105976 ram
less than 70 quid. Bung a cheapo case if your psu is no good say 20 quid and jobs a good un. Not really an upgrade though lol
http://www.ebuyer.com/product/86501 case 14 quid 84 quid plus delivery the job