Programs to improve your computer's performance

Hello

Please can anyone help me work through this minefield! My computer  (running 
Windows XP SP2) is about 3 years old and, although I am very satisfied with 
it, it obviously does not run as quickly as it used to. There seems to be a 
wealth of stuff you can buy to supposedly improve your coputer's performance, 
but how do you know what to choose and whether it works?  I have  reputable 
anti-vrus software installed, plus the regular updates from Windows, such as 
the malicious software removal tool. However, people are telling me that I 
also ought to have a spyware removal tool. I have also considered purhcasing 
a registry optimizer because I believe corrupt registries cause lots of 
performance problems. 

I don't feel confident enough to start tinkering with my computer myself, 
and, although it's irritating to have to keep shelling out money, I am 
prepared to purchase anti-spyware and registry optimizers, IF they really do 
what they say on the tin. Can anyone advise?

thanks v.m.
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