If you can't afford new components for your PC then you can still improve the performance without spending any money at all. Here's how to overclock your CPU whether it's an Intel or AMD processor. See also: AMD vs Intel.
In brief, overclocking is the process of making your CPU run faster. It's a bit like tuning a car's engine and remapping the ECU to extract more performance. The performance is already there, but by default, the manufacturer restricts performance to a 'safe' level to ensure it's reliable for a longer time. You can choose to make your CPU work faster by changing a few settings, but there are risks - see below.
The speed of your CPU is calculated by taking a base clock rate, eg 99.8MHz, and multiplying it by the, er, multiplier, which could be 34. This equates to a clock speed of 3400MHz, which is 3.4GHz. You can increase the clock speed by changing the base clock but it's more common to adjust the multiplier. You can only do this if you have an 'unlocked' CPU (see Which processors can I overclock below).

How to overclock CPU: Warnings and risks

Before jump into doing this, there are some warnings to consider. For starters, overclocking will almost certainly void any warranty on your machine – although some are built with overclocking in mind. You may damage your CPU, or even other components by overclocking so you will be taking a risk with yourself responsible.
See also: Acer Revo Build hands on review - the modular PC.

How to overclock CPU: Before you start

Since overclocking involves pushing your CPU beyond its factory settings, you should monitor it. We recommend you download and install software such as CPU Z in order to do so – it can measure and display a real time measurement of each core's internal frequency and memory frequency.
It's also a good idea to check all your fans are working properly, especially the one cooling your CPU heatsink. Pushing the CPU harder creates more heat and if you don't dissipate that heat you risk damage. Making sure your computer is clean inside will help to keep it as cool as possible.

How to overclock CPU: Which processors can I overclock?

As a general rule of thumb, the higher-end processors are approved for overclocking. Not every CPU can be overclocked so you'll need to have one which is unlocked in order to manipulate the base clock rate or multiplier.
Intel's Core i7 Extreme Edition is unlocked and so are the latest 6th-generation Core i7 and Core i5 chips. If you're not sure then Intel has detailed specs here or you can simply attempt to overclock and you'll soon find out.
AMD's latest FX range of processors come unlocked as well as some Athon models, too. Once again, if you're not sure then you can simply try and overclock and see what happens rather than spending forever researching your particular chip.
AMD OverDrive

How to overclock CPU: Software or BIOS

If you're trying to overclock an Intel processor you can download the Extreme Tuning Utility (Intel XTU) software in order to do this. It provides access to the settings you need to overclock such as power, voltage, core, and memory.
On the AMD side of the fence, you'll need to download OverDrive (above) which is effectively the same thing. It includes automatic tuning if you're unsure of what you're doing, control over your fans, monitoring and also stability tests.
If you'd rather not use software then you can go down the old-school route of using the BIOS. Boot into the BIOS using the correct F-key and find the section where you can change the settings of your CPU (every BIOS is different but it shouldn't be hard to find). Now change the multiplier up by one, save your settings and boot the computer.
If your system runs smoothly and stably at the new speed then you can try a push it further. If your PC boots incorrectly or you experience any performance issues then you probably need to decrease the multiplier back down to a stable level.