I field a lot of questions regarding slow computers. It has been my experience that if you have a sudden slow down in your computer you need to check and see what was recently installed and you should also check and see if you have a virus or not. One of the companies that I have always trusted to scan and detect infections is Symantec. They have a free scan you can preform to see if you are infected with anything. Here is the location Click here: Now continue to click on: "Continue to Symantec Security Check" and not the "Download now". This opens a new window and you can click on Start under the Virus Detection section. This will start some programs that you will need to let run.
Once your scan is complete it will give you results of the scan. You should have a better feeling now if you have no infections. If you do get some infections listed then it might be a good idea to try and remove them. This is were we professionals come in handy because there can be a lot of variables involved in removing the infection(s).
Most people don't notice their computer slowing down until it is a crawl and most times the best course of action is to reload the operating system. The clean install is the most effective method in getting that computer back to brand new speeds but doing this can be a pain. If you don't have a copy of the original disks that came with the computer it makes it very difficult to get back to original form because of missing drivers and programs.
If you have done the scan and you have no virus check the space on your hard drive. If there is enough room on the hard drive then check your memory and see if you have enough. You can upgrade your memory and sometimes you might be surprised at the increased speed.