Clutter

• Over time your computer will get cluttered with useless files you no longer need, programs you no longer use and processes you may not even know are running. There are many things you can do about this problem. One good way to reduce clutter is to defragment your hard drive. Windows offers the free Disk Defragmenter tool in Vista, 7 and some versions of XP. You can also download third party programs such as CCleaner to defragment your hard disk, many of these programs offer other cleanup functions that will help with your clutter.
  
  

Outdated Hardware

• With technology advancing so fast, it wouldn't be uncommon for your computer to have some outdated hardware under the hood. CPU's, graphics cards and many other pieces of hardware may be designed to do less than you want to use them for. Even devices that aren't directly related to performance can affect it. For example, if you have a poor network adapter, it won't necessarily have anything to do with overall PC performance, but will affect your Internet performance.
  
  

Other Malware

• While you may have run a virus scan on your computer to convince you a virus isn't behind your system's poor performance, it may be other forms of malware (malicious software) causing the slowdown. Adware and Spyware are common causes of slowdown. Like viruses, the user usually doesn't purposely install these programs and often doesn't even know they're present on their system. Make sure your Anti-Virus has spyware and adware protection before ruling this out. If not, there are many free malware removal tools that don't necessarily deal with viruses.
  
  

Not Enough RAM

• One of the main reasons for system slow down, not having enough RAM, becomes a common problem on older systems not specifically made to run newer software. According to LaserChips.com, upgrading your RAM is one of the things you can do to increase a computer's hardware performance. RAM is used to run processes and programs currently open and if you have more programs open than you do available RAM, you may get anything from slowdown to error messages.
  
  

Security Software

• According to DownloadAtOz.com, security software can, ironically, be a big cause of system slowdown. Some security vendors see security as a priority over performance and while this may be true to an extent, there's no need to bog down your system with a poorly designed security suite with many light and resource-friendly options available. Many of the resource-hogging security programs are technically the best at doing what they do, but what's the point of having a safe computer if it's too slow to use.