The problem is there are so many little computer problems that can make your computer lose performance slightly that you don't necessarily notice it until there is more than one thing affecting it.
Full hard drive
If your hard drive is too full then there isn't enough room for temporary files, or virtual memory. The size of hard drives today makes this less likely, but its still may happen.
Not enough ram can make your computer go very slow. There might not be one single better thing to speed up your computer than upgrading your ram. This will give you the most noticeable difference in speed. Check my post for more information.
SpyWare and AdWare
These are not only annoying but can seriously affect your computers performance. Luckily there are plenty of free and top of the line programs out there to help you deal with this. If you constantly get pop ups, or go to websites that you did not intend to, you probably have spyware, and even if this doesn't happen it may be there. Check out www.Ad-aware.Com for a good anti-spyware program.
There are a few things that should be done regularly to keep your fast computer from becoming your slow computer. Disk defragmenting, clearing temporary files and other needless files through disk clean up need to be done often and can be scheduled to run at certain times. You should remove any unused programs.
Clean up your Registry
After a while through no fault of your own and because of malicious programs, your registry becomes a mess. Fortunately again there is software for this as well.
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