There is a very simple way to make sure that one's computer runs fast regardless of its age. Use it for what it is needed for and nothing more. Don't have a super hyped up machine if all it is going to be used for is to look at email and dirty pictures. Any computer made after 1999 or so would be able to handle that with ease, as well as writing documents, PowerPoint presentations, Quickbooks, and many other programs all at once.
It is very simple: Don't load programs that are not going to be used.
I installed a wireless router the other day and it said that I needed to install the software on the CD first. I tested this by not doing so, and what do you know: It worked!!!
Many products will come with installation Cd's that don't in fact have the drivers on them but instead just superfluous software that runs in the background reminding you that you have their product. Almost every time, all one needs is the driver.
Just like how these "malware" programs run in the background, so too do the programs that one uses on a daily or weekly basis. Whenever a program prompts for a shortcut on the desktop or quick launch bar think about whether they would be used or not. If not, don't let the computer make the shortcuts. Also it is a good idea to disable most things in the start up tab under msconfig.
Viruses will kill a computer and wipe out all of the files if it is not protected by a trustworthy anti-virus. So many people ask me why their computer stops working or crashes or gets horribly slow AFTER installing anti-virus. The answer is always this: they chose the wrong anti-virus. Without exception the best anti-virus program for the past 4 years has been AVG. It even has a free edition that works just as well as the paid for version. It updates daily and when prompted, is a small program that works in the background with out slowing down the computer, and catches alot more than what the others would catch. In 1994 Norton began to be the biggest most effective antivirus software. It was displaced around 2001 or so by McAffee. Now both those programs have lost their effectiveness. They do not catch the REAL viruses, they slow down the computer and they are almost impossible to remove completely. They have in fact become malware themselves. AVG working in unison with Spybot and AdAware is what so many computer guru's have been recommending for the past 5 years.
Finally, if all of this does not speed up the computer that is currently running slower than sap dripping up a tree in the arctic in slow motion, then the best choice is a clean slate and wipe the hard drive out and reinstall the operating system. Back up the information that is needed and just bite the bullet and it will be like a brand new machine.
I guarantee it.