A computer virus or some other unscrupulous ad-ware or spy-ware programs may have invaded your computer.
At best these types of programs can assault you with endless advertising while you try to browse the Internet.
At worst they can hijack part or all of your Internet connection to attack other computers on the Internet, or steal your private information, like account logins and passwords for your online banking. They can even be destructive to the data you have stored on your computer so you might lose that report for school you worked on for 3 weeks, or the only pictures of your daughters wedding you just moved from your digital camera.
Detection and removal of these types of programs is of the utmost importance but, this can be difficult for the average computer user due to the multiple techniques employed by these programs to avoid detection and removal.
Normal usage of your computer creates numerous extra files on your system.
Installing and removing programs leaves little bits of their files all over your system as well. Internet usage is worst of all for cluttering up your system. All these things combined can cause file fragmentation and slow your system to a crawl.
If your computer was purchased more than two years ago, 80% of the latest software available today, will probably run painfully slow, if it will even install. If you visit any of the big computer retailers, they'll agree. They will also tell you that you need a whole new computer. Normally, with upgrading and reusing most of your old hardware, you get the equivalent of a new computer for a fraction of the cost.