Depending how old your PC is (the specs), and what you use it for.
If just normal browsing etc, one of the cheapest, and easiest upgrades is RAM, and usually quite effective. This is particualrly true of XP - you can run it on 128MB RAM, but it is very slow. M$ recommend 256MB minimum, and on a 500Mhz chip, it is resonable, but just ensure you disable the unnecessary startup items. Ideally you want 512MB or more in my experience. Choosing programs that use low resources (like firewall, AV and anti-spyware) is a prudent route to take. Basic things like a good clean-up with a program like CCleaner can help, and defrag
I wouldn't spend too much if you are thinking of getting a new PC if the future, as you will not get much of your money back.