OK
[ What details do we know about your PC now, its got enough ram and it has a large hard drive.... and it is slow.. and you want it fast.]
I assume though that you want to do it for very little or no cost with the exception of your time. this is understandable.
Correct me if I am wrong , what you want to do is make a slow PC as efficient as it can be, getting all the power it has to mussel up to your game
There are a few things you can do
#1 .........no matter what op-sys you are using, reinstall it and strip it down to its lightest weight. get rid of all the non gaming components.
Deleting or Un-installing previously installed programs like office, old games, etc will not work. Your registry will have to much junk in it and so will the rest of you hard drive. you must reinstall your op-sys. (Be sure it is new and stripped down.)
Doing just this could make your slow PC more than 100 - 500 % more efficent., depending how dirty and bulky it is maybe more. Meaning to you.............. A Faster PC ! !
#2... this would depend on other unknowns, that being your hardware. That is to hard to guess about but you probabley will post that as your next question and get more answers. That said the next thing to do is get that info and ............ post it for your next question
Most of the answers here are pretty good, but we are not going to change any hardware yet and that leads us to back to software.
Just as I said Perfectest says " Microsoft Registry Problems = Slow Computer! " but here is the problem....
Registery cleaners often dont work as well as planned. The registry is very complex and critical to your pc's performance and the sligthest error can cause grave problems or even disable your pc completely.
Registry Cleaner programers know this too and to protect there products good reputation they don't make it do things that risk causing you problems. Things like really get your registry clean and efficent ! Some of them are ok, in that they clean know problems. but .........
If you want a really clean and efficent registry "Re-Install your op-sys"
By the way, there is a program (don't recall its name) that will assist you in removing what is normally not removable Windows XP components. With these components removed your pc dosent have to spend time searching through them, that means more efficency and that equals more speed.
You could think of it like this:
The smaller the jockey, the faster the horse can go
Another advanced trick you can do is to partition your large hard drive, but that is yet another story
Do it, you will be notice the increase in performance
Good Luck