Now that computers are reaching a level of performance we are unlikely to exceed quickly, we may be keeping our computers in operation for more than just a couple of years. As a result we may actually need to maintain them in order to ensure maximum useful life.
As time goes on your your computer's files can become disorganized, more and more unneeded and unwanted applications have been loaded and forgotten, and viruses & malware will have made their way to your hard drive. Any one of these things can slow your computer, but in combination they can render the computer virtually unusable.
First, go through you documents and get rid of those things you don't need. Run disk cleanup monthly and defragment your drive every six months or a year.
Go to Start->Control Panel->Add/Remove Programs and go through the list. Remove any programs you don't think you need. This will free disk space as well as remove any memory resident software that might be part of the application, improving performance even more.
Windows Update service is a free, regular service that will send updates to your computer for the OS and Internet Explorer. Security patches are the most important part of this service, which you can configure to download and install updates automatically.
Those fans you hear humming inside your computer are like 24/7 HVAC systems. The fans cause a constant parade of particulates to get sucked into your computer - and no matter how clean you keep your house there is always some level of dust in the air.
Dust and heat are the most likely cause of computer failure. Dust is the biggest enemy because as it coats the inside of your computer it will cause heat buildup. Heat shortens the life and can even destroy components in the computer.
You can pay to have the computer cleaned or pick up what you need at any computer store. As a matter of fact, the local department store has what you need.
To clean the inside of the computer you will need to remove power from the computer, vacuum the dust buildup on the fan intake(s) and, if you're feeling really ambitious, remove the cover, and vacuum the inside as well as blow it out with canned air.
In addition, you can use some a lightly moistened cloth or lens cleaning wipe, followed by a dry dust cloth. Be very careful to avoid scratching the surface of the screen.