As a working PC tech, this is one of the most common issues that I face day in and day out. Although it happens to varying degrees, everyone at some point realizes that their computer is not performing like it used to. To some the slowdown process is gradual, to the point where they hardly notice until someone else uses their computer and complains. For others, it happens very quickly so that there is a sudden and unmistakable loss of usability. Whichever is the case for you, here are some general guidelines to help you in diagnosing the cause (or causes) of the slowdown.
The first thing I suspect when someone tells me that their PC is running slowly is that they may be dealing with adware or spyware. Typically, a computer infected with this type of malware will display other symptoms as well. Odd behavior such as pop-up ads, your internet home page changing, sudden closing of your browser or even system crashes are all symptomatic of a Spyware/adware infestation. Whether or not you are experiencing any of these additional symptoms, you should scan your PC with a good antivirus/antispyware program. My personal favorite is AVG Internet Security and I highly recommend it. Whichever program you use though, just make sure it has a good reputation.
Another common source of system slowdowns is a highly fragmented hard disk. Disk fragmentation happens over a period of time and may happen so gradually that you don’t even realize how poorly your computer is performing. Fragmentation usually doesn’t cause other symptoms, so if your PC is otherwise acting OK then fragmentation may be the problem. Windows has a built in defrag utility, however it will not defragment especially large files and some system files such as your Paging File (sometimes called the Swap File) or the Master File Table. To run at peak performance you should use a third party defrag utility such as Diskeeper. Third party programs ‘one-up’ the built in Windows utility because they have the advanced features that allow you to defrag the entire drive.
Lastly, another reason many PC’s start to lose performance over time is that too many programs are running in the background. This happens over time because software developers assume that you want to run their application every time you’re at your computer. So for your “convenience” they helpfully add their app to the startup group. That’s fine except by the time you’ve added 30 or 40 new programs to your computer, you’ve got 30 or 40 programs starting up automatically every time you boot your computer. With all these programs hogging precious resources from your system, there isn’t much left to do the things that you want to do.