A computer sometimes slows down because it does too many things when it starts up. On a Windows computer, click the Windows "Start" button, type "msconfig" into the Search field and press "Enter." Click the "Startup" tab. Remove any non-essential programs and your computer should function better when it starts up and afterwards, because it will have less programs running in the background.
On Macs, go to "System Preferences" and select "Accounts." Check the startup and login items and remove unnecessary programs.
Also, when you have too many files saved on your desktop, it slows the computer down as soon as your desktop loads. Move the files into the appropriate folders and into a directory on the hard drive instead of leaving them on the desktop.
Malware, viruses, spyware and other types of malicious software all slow down computer processing. Make sure your anti-virus software is up to date, and run a complete scan of your system. If you don't already have a paid service, subscribe to one or download a free protection program such as Microsoft Security Essentials or AVG.
It is possible to have too much of a good thing. If you run more than one anti-virus program simultaneously, the resource drain on your computer could slow it down dramatically. You should only have one anti-virus program.
Problems on the hard drive slow a computer down. Operating systems come with utilities that scan and repair various types of issues that occur in the file system. Running these at regular intervals is a good part of overall computer maintenance, and can speed up the performance of your computer. On a Windows machine, run the Disk Defragmenter, the Disk Cleanup utility and the error checking utility. This program, called CheckDisk, can be accessed by clicking the Windows "Start" button, selecting "Computer," right-clicking the hard drive and selecting the "Tools" tab.
On a Mac, run Disk Utility to search for errors on the disk and to repair permissions. If you find any disk errors, use OSX Recovery to repair the errors. Press and hold the "Command" and the "R" keys during startup to access the program.
Deleting unused files or moving media files to an external drive can also improve performance, especially if your hard drive is almost filled to capacity.
Because so much of what most people do on computers involves the Internet, always consider Internet-related causes when you notice a slowdown in machine performance. Delete temporary Internet files, clean your browser cache and ensure that you have all the latest browser updates to speed up a computer that acts slow when you access the Internet.