You probably can, you're simply making the processor run above it's factory specification. Some chips overclock very well, other do not.
First thing I would do is download CPU-Z which will tell you what chip and motherboard you're running, then do a search for that particualar chip and motherboard. The directions vary by motherboard, you may not even be able to do it. The important thing is to make sure you're within some acceptable levels for temperature, you should completely dust out the system before proceeding.
The I wanna try it right now directions:
First be aware you can fry your chip doing this. Reboot your computer, hit delete (might be something else here but we want to get into the BIOS). Once in the bios find where your CPU temp is listed, if your at or below 40 degrees you should be good to go. That is conservative temperature, you may also have some software for monitoring this through windows as you will want to do that once you overclock it. You don't have to monitor it 24/7 but you want to check it initially, once temps get up into the 60's you're pushing it at least for my limits. It varies by chip.
Once you have determined the temperature is in a good range find the setting labeled CPU frequency (this will vary on motherboard manufacturer). Increment it by a value of 1 or 2. Save and reboot, you're now running a faster machine. You can keep incrementing this until the machine starts crashing or the temps start getting real high. The higher you go the faster it will run.
That's the basic step, there are of course many things you can do that go beyond that to get a faster clock. Just remember one thing heat kills a CPU and this will most likely decrease the life of your CPU. Having said that 've been running my P4 at 3.3 for about 3 years and at one point had it up to 3.5.