alot of advice here... SSD will speed up your computer but there pricey
if i was you i'd do a bit of trial and error and see how things go starting with the cheapest option first.
1. format your computer
2. install a x64 OS ( id get win7 if you can buy/get it try windows 8 there a free beta
http://msdn.microsoft.com/en-us/windows/apps/br229516 )
3. add extra 4gb of ram (approx £15)
if your not happy with the improvement look at getting a new processor
