DaCostaBR: It's my mother's computer that she's complaining is way too slow
Formatting probably isn't what you need for this. I've "fixed" slow computers a bunch of times for work and family, and formatting rarely fixes the problem, though if you've got additional knowledge of why you want to format, then by all means, have at it.
If her computer is running slowly, and she's got antivirus running, then it's most likely a case of a bunch of background processes eating your CPU or RAM. Bring up Task Manager and see; if there are a few obvious culprits, fix them directly; usually, that means uninstalling(and perhaps reinstalling) them. If you know how to inspect a registry, even better. Check your hives for failed installs that keep trying - and failing - to run. I just had to clean about 1,000 entries for a family computer because they had four duplicate hives trying to install a software suite, simultaneously. Made things horribly slow.
If you think that you've got some sort of malware, then formatting still isn't the best option, because a format doesn't actually wipe the information, so you can (and I've seen this several times) format a computer, do a fresh install, and if everything is pretty much the same, you can still have the malware resident on your computer. There are parts of your hard drive that a normal format doesn't erase, and even erased data on a disk is still there until it's overwritten...
...which brings me to my third advice, which is find and use a zero-fill tool if you really want to wipe everything away and start from scratch. It's the "nuke it from orbit, it's the only way to be sure" approach, and is rarely the best fix, but it is also pretty easy. I like
DBAN myself, but it's hardly the only option out there. Doing this takes some time, but if you want to get rid of a computer that once had useful personal information on it, and also don't want to smash the hard drive with a hydraulic press, it's a wise step to take.