You must have seen the ads (PCMatic for example) The ad promises to increase your computer's speed by "cleaning" the Registry, the hard drive, and other less mentionable parts of your computer.
Beware. Most, if not all, of these programs are malware, especially the free ones. Before downloading any such program, Google for reviews of then program's name. See what the rest of the Internet thinks of them. Usually you will hit some reviews that call the program a virus. I don't run freebie programs of any kind unless I find solid and unanimous good words on the net about them.
Plus, you can do it yourself, which is safer. Running a little known program on your faithful computer is hazardous to its health.
Keep things clean. Your computer came from the store with a bunch of games and craplets that you never use. Zap them. Do "settings" then "Control Panel" and then "Add and Remove Programs". Remove anything you know you don't use. Be careful about things you don't know about, programs with important sounding names that you don't recognize. Some of them are vital parts of Windows. Some of them might be virii. Google these programs, but interpret the answers with care. There are a bunch of websites out there that will respond with a means nothing answer to any program name, and then attempt to sell you on their Computer Fixer Upper program. Ignore those. Look for answers that come from people, not programs.
Make a list of the questionable programs, and keep it to hand.
Run Task Manager. It comes with Windows, and pops up when you hit control-alt-delete. Look at "applications". That's programs that are running and showing a window on your desktop. You should recognize all of them. Any application that you don't recognize is very suspicious.
Check the "Process" window. Processes are all running programs, most of which don't show windows on your desktop. Programs in the Applications window will also show up in the Process window. Any live virus on your machine will show up in the process window, probably under an assumed name. Processes claiming unusual amounts of RAM or CPU time are suspicious. It's usual to have 30 odd processes running, but much more than that is suspicious.
Questionable programs from the Add and Remove programs applet that DO show in the Process window are more likely to be a necessary part of Windows. Questionable programs that DO NOT show in the process window are more likely to be junk you don't need.
Once you are sure you have identified something as virus or just plain junk, get rid of it. Start house cleaning with Add & Remove programs. If that works, it usually does a better job than anything else, it can remove the programs Registry entries and it's disc files. If Add and Remove programs doesn't work, go after the offensive program's disc files with Windows explorer. And then go after the program's Registry entries with Regedit. Delete all registry keys containing the offensive program's name.
Secret. The name the program shows in Task Manager may not be the program's disc file name. In which case, a search of the Registry with Regedit, looking for the offending name in Task Manager, will often turn up a key that gives the program's disc file name.
Be careful. It's is quite easy to zap a necessary part of Windows which is a catastrophe.