A slow computer can be a sign of an overly fragmented hard drive.
Your computer saves parts of a file one place and other parts somewhere else, based on available space.
Over time, this fragmentation builds up, forcing your computer to look in several different places to read a file, which slows things down.
Defragging places your files in adjoining blocks and makes the most frequently used files available for fastest access, which improves performance over time.
It’s ideal to run a “defrag” utility once a week or at least once a month.
Note that if you have not defragged in a long time it could take a few hours to complete.
It’ll go faster next time you do it.
Look for the Disk Defragmenter utility included in Start | All Programs | Accessories.
Windows ships with various services that automatically load when you startup your PC, but this can make your start time sluggish.
Check out what starts automatically and set the startup of items you likely won’t need to "manual."
To do this, choose Start | Control Panel, and then go to Administrative Tools | Services.
Right-click items and choose Properties to change their automatic startup setting.
Be careful not to change the startup setting of any services that are needed for networking or applications you use.
Spyware, viruses, and uninvited clutter in your Windows Registry and file system are one of the most common causes of a slow PC.