If you look at the spec and the age of your machine I think you will realise that upgrading the processor is not a financially viable option. An upgraded processor will not give you a huge boost in performance, the only processor you could get will be a used one (probably from ebay), the board also uses DDR memory which is very expensive nowadays. In short you would be better advised to look for a more up to date PC. Those using AMD processors are generally cheaper than those with Intel processors and you should look for one with DDR3 memory and one with built in graphics to keep down the cost. You could also consider building you own using a motherboard bundle from ebuyer or novatech or similar company, re-use your existing case, power supply and drives.
Best Cyber Monday deals 2015 live blog: Best UK Cyber Monday tech deals, sales, discounts & offers