A slow computer is generally like that because it is lacking resources, be it hard drive space, memory or processing power. There are general things you can do to alleviate these problems, and there are also specific things that can be done to different versions of windows, none of which require money or extra hardware.
GENERAL
1. First step is to always get rid of all useless stuff on your system, rather than work around it. If you are running windows XP, go to:
Start > Control Panel > Add/Remove Programs.
If you running vista 7, go to:
Start > Control Panel > Programs and Features
Go through each of the programs and uninstall all programs you do not use.
Note: Do not uninstall programs if you do not know what they do, first find out if they can be removed without causing problem to your computer. Don’t just leave them though, as it will be wasting space and resources.
If you have not used a program for more than two months, you may not need it. If you may need it in the future, back it up with all saved files and uninstall it, you can always install later but until then, it is just slowing you down.
If you are running a firewall/antivirus program that you are not paying for, such as Norton, Panda, McAfee etc, these will generally be bloated with features you don’t need and fancy interfaces which will be slowing your system down. Uninstall them and replace with other antivirus like Avast.

2. Go through all your files/folders, ordering and tidying them up. By ordering, you will find files that are outdated and no longer needed and can be removed, as well as making your file system more efficient, so it takes you less time to find what you need.
3. Download, install and run Ccleaner. It is a program that gets rid of cookies, temporary files, and the junk you don’t see.
4. Download and install Defraggler. The defragmentation software that comes with windows is not good enough. There are paid suites that can do the job much better than free ones however the free ones are much better than the windows ones.
Run Defraggler 3 times in a row. The first timer it tidies it up and gets rid of most fragmented files, the second it condenses most of the files, freeing up extra hard disk space, the third time it does a final “polish” of your system.
5. Download and install “Soluto”. It is a program which finds out what programs load at during your boot sequence and determines which can be stopped from loading. This will reduce the boot time, as well as using less system memory as your computer loads.

Tuneup is a nice pc maintenance tool too.

mechanicaldummie: Tuneup is a nice pc maintenance tool too.

Yes you are right. There are many other maintenance tool too but i just cant mention all in the post.