I don’t know this game, and I also don’t know your computer specs, but as sandy said, you can at least run it, so it can’t be that bad.
There are two ways to go about this:
1) with what you have, lower the requirements in the game itself. There must be an “options” panel somewhere, and you can change different things, for example “graphic details” could go to “medium” instead of “highest” and it will run a lot faster, possibly without limiting the visual quality much. Similarly, you could change the frame rate, or remove anti-aliasing, depth, or, above all, resolution (try 800×600 for example, I’m sure the game will be much faster in that without having to change anything else).
If that doesn’t work, or if the graphics are now not good enough, you could pay some money and go to step 2.
2) the best thing to buy is a graphics card in this case. There are many cheap cards that do the trick, and if your system is relatively old, you can probably get a much better card than the one you have for under $50. But of course you need to find out what card you have first, and also someone to install it if you don’t feel confident enough (they’re actually very easy to plug in, but you just have to make sure you’re careful not to scratch or break anything while you’re inside the computer case).
Another thing that might generally help with your computer (though I doubt it has anything to do with the particular game being slow) is RAM. Over the years I have discovered that simply adding some RAM solved most of my problems (old Win95 crashing, games freezing, slow performance when multitasking etc). Most newer systems have more than enough RAM, but if yours has under 1GB, perhaps you should add some. The prices there have also dropped.
3) if all else fails (and if your computer is some 20th century antique), just buy a new one, making sure to invest more in RAM and a graphics card than a processor. All CPUs are by now so fast that it doesn’t make a difference unless you’re analysing SETI data. Oh, and one last tip: never buy the “cutting edge” stuff. It costs twice as much as the previous model and is not twice as good. Always buy the 2nd or 3rd best choice, they are better value for money. Either wait for a new model to be established for a couple of months (in which case the immediately previous one will drop to half price) or just buy the one before that. This goes for all new technology (gadgets, processors, RAM chips, DVD players etc).