Over time, the speed of computers with Microsoft Windows can  decrease. This appears as the system taking more time to respond to a  user’s actions like opening files, folders, surfing the Internet and  other tasks. However, there are things you can do to speed up your  computer.
The following is a list of ways you can improve your computer's  performance. These steps are intended to be used in order. The key is to  do the simplest and least invasive thing first until your system starts  responding better. 
Golden Rule: Do No Harm
If you want to improve the speed of your computer, that’s great.  However, above all, do no harm and beyond the scope of this Windows  forum, ask for help from those you know if you get in over your head.  Remember: you have time. You can stop most processes I discuss without  causing harm. Caution and common sense go a long way when working with  computers; problems are often much easier to fix than they appear. 
Steps to Speed Up Your Computer’s Performance
1. Make Sure Your Hardware is Sufficient
Above and beyond everything you can do with software to optimize the  function of your computer, making sure you have the proper hardware to  support Windows is critical.   
2. Clean Your Desktop
Is your Windows Desktop dotted with files? Have you noticed that your  computer has been running slower and slower? Do you see the hard drive  light often flashing while you wait for the computer to respond to an  action? There are steps you can take to fix it. Read this article.
3. Scan Your Windows System for Errors
An operating system is a collection of files that perform different  functions. It is possible, over time, that one or more of these system  files has changed or become corrupted. If this happens, the speed of  your system may decrease. By using a utility called “System File  Checking”, it will inspect these files and correct any problem it finds.  
4. Scan for Viruses, Spyware and Adware
Every Windows computer is vulnerable to viruses. Viruses are nasty  little programs that cause both major and minor problems for users. 
Spyware and Adware are programs created by companies to find out more  information about customers, so they can better market products to  them. Usually these programs are not created for malicious purposes.  Spyware and Adware can load into computer memory and slow it down.
Periodically scanning and removal of  Viruses, Spyware and Adware is a great way to improve computer performance.
5. Uninstall Unused Programs
Over time, you may have accumulated programs on your computer that  you do not use. When a program is installed, it creates connections  between the program and the operating system. Even if you don’t use  these programs, they can slow down your system. If you have the original  program’s installation disk or file, removing programs will free up  space on your computer and may speed up your system's performance.
6. Adjust Visual Effects for Better Performance
Windows provides a number of interesting visual effects including  animated windows and fading menus. If you do not have enough operating  memory (see above number 1), these effects can slow down your computer.  Adjusting or reducing visual effects can make a difference. 
To speed up Vista computers in particular you can disable features and function that are not used.   
7. Don’t Automatically Start Programs
Programs use operating memory (RAM, also called system resources).  Unless you always use these programs, you can keep them from loading at  Windows startup and speed both the booting of your computer and its  performance. 
8. Defragment Your Hard Drive (After you have tried everything else)
The more information and programs you have on your computer, the more  it is likely that Windows has placed parts of the same file in  different locations on your hard drive. It may do this for hundreds of  files and programs, depending on how much space is available.  Defragmenting the hard drive places all information for each file in one  place. The result can be a faster computer experience. 
Important Note: Before you even try this, backup all your work  onto a different computer, hard drive, CD or disk. Also, you should  only do this if you have a reliable source of power for your computer.  Doing this when there is a brown out or power problem in the  neighborhood, is not a good idea. If a computer turns off while it is  defragmenting a drive, it will create more problems than not  defragmenting it in the first place. 
9. Reinstall the Operating System and Programs
If you have done all the above steps and your computer does not  become more responsive, you might consider reinstalling the operating  system and programs. This is a scorched earth option - basically you  backup all your data and user settings, wipe your hard drive clean and  re-install the operating system with your original disks. Next, install  all your favorite programs and restore all your data and user settings.  
The nice thing about doing this is that you essentially have a  new computer, free of years worth of software and registry changes,  additions and errors that can turn a fast rabbit of a Windows computer  into a slow turtle. The bad thing about this choice is that it takes  time and planning to do it correctly. If your PC is 3 to 4 years old and  continues to run slower than it did when you purchased it, this might  be the only action that will make a significant difference.