Not Enough Hard Drive Space or Memory
Although this is unlikely unless your computer is very old, it is possible that your computer may not have enough hard disk space or RAM to keep your computer at top speed.
Earlier operation systems such as Windows XP required at least 300-500MB of free space just to allow for basic disk activity, whereas Windows 7 requires a minimum of 16-20 GB of hard disk space just to run the OS. Needless to say, the more applications installed, the more space required. To free up additional space, you can delete any unused files, purge your temporary folders and uninstall any applications that you don’t need or use. If nothing works, it may be time to go in for a new hard disk drive.
Memory, like hard drive space is subject to change as time goes on. While an old laptop running on Windows XP would probably require at least 512MB of RAM to run software efficiently, newer machines running on Windows 7 and Vista would require at least 2 GB RAM, and 64-bit version of the OS may require 4 GB of RAM to take advantage of the extra power. If you play games, watch live videos or use other rich multimedia content, you’ll likely need even more.
Another related issue could be a fragmented hard drive. As files are created and deleted, their locations on your hard drive may be scattered in a way that doesn’t allow for fast access. You should regularly run some type of defragmenting software (either the one built-in with Windows, or a third-party application) to keep your hard drive at its best possible performance.