Really? No info? Just "why is it going slow"? How can anyone answer that? You need to give information. What are the specs (right click on "computer" and click on "properties")? How old is it? Do you have antivirus? If so, which one do you have? How often do you download things? Do you download torrents? What programs have you installed recently (just before it started getting slow)? What color is your shirt (jk)? There are too many factors to consider to be able to answer that question without more info.
The reason your desktop is moving slow is because it was designed to be attached to your desk. It is usually held in place with screws so it should be easy to detach. If you are talking about your computer, then the problem could be a corrupt hard drive, too many programs loading while booting, a virus, or simply old age. I use Advanced System Care from Iobit but without more details it is hard to diagnose your problem.