I recomend that firstly If not already done, upgrade your ram, increase of ram will increase speed and performance,
its easy to do yourself,
you can download a tool from crucial
http://www.crucial.com/uk/index.aspx
this will tell you what ram you have and what you need to upgrade, their ram is compatable, and much cheaper than original, I used on 2 previous computors and pleased with the results,
my old sony lap top I upgraded ram, and was running vista, later I upgraded to windows 7, which gave further improvement, but I found my printer software wasn't compatable on w 7,
I got my windows 7 upgrade at a pre release price offer , I wouldn't have considered upgade at the full price, you have to weigh up if your pc is worth it,as pc spec is soon out of date these days, ram is definatly worth a try,
how far do you go,? although you can upgrade ram, and OS the rest of your computer components will also be out of date, whats next? mother board, hard drive, by the time you finish you could have bought a new computer with all you need