There are several reasons for your fast computer to become a slow computer. Your operating system might have too many programs installed and the registry has become corrupted and needs cleaning. Every time we install and later remove programs the registry can become corrupted. Other problem areas might be more RAM memory needed for some newer programs your running, your hard drive might be getting full, your hard drive might be corrupted or fragmented, you might have a virus, or some malware infection.
The registry always needs cleaning and correcting. It holds all the configuration information for your operating system, all installed programs, and hardware devices. Sometimes when you remove programs file pointers are not removed or reset and still point to files that are no longer installed. When you start your computer the operating system slows as it looks for files to load that are no longer on your computer. There are many good registry cleaner programs available including some free ones that should help. I usually look at CNET download.com and read all the user reviews to find the better ones. Right now Registry Mechanic seems to be the favorite. I also like Obexo which is a free registry cleaner.
Ram memory is easy to upgrade and not very costly. If you have a notebook, there is a cover plate on the bottom to get access to the memory. There is 2 clips that hold it in place. I have a YouTube video that takes you set by step.
You should also make sure you have a top rated anti-virus program install and updated daily. One of the best is AVG and it’s free for personal use. You can download it at the link provided.
Another problem might be malware, spyware, or data mining programs that get installed while we are surfing the Web. They are easily remove by a top rated free program called Adaware (WARNING: not Adware) This program is also free for personal use.
The hard drive can also be causing problems either through corruption or fragmentation. There is another free program called Smart Defrag that will keep your drive running at its optimum.
I also have another YouTube video showing how the hard drive acts when fragmented. 
If you address each one of these problem areas you should notice a considerable speed improvement. If you want to learn more about computers you might also check out my Computer Repair Training series.
Slow Computer is a website where might find some helpful information.