First off,use Malwarebytes,it's free (With an optional premium version) and I have used it for a day now,I know,it isn't a long time but many others can tell you it's a good program.
http

/www.malwarebytes.org/mwb-download/var2/
No,I haven't used this program yet,but it's also by Malwarebytes,it's a Anti-Rootkit program,which you probably have since you lost 120GB`s,keep running both of these program's if you choose to install them.
But be warned,Anti-Rootkit is in beta,so I suggest you read the warnings and decide for yourself if you want to commit.What it does is remove a Rootkit (I suggest you look it up) which has changed files and replaced some that involves your operating system.The reason why I suggest you read the fine print is simple
If you use this program,and you indeed have a RootKit and this program removes said RootKit,it will most likely remove important files that involve your operating system,causing errors.That being said,this program comes with a feature that will try and replace those files so you computer won't have these problems.Many people have used this program with success and have had no hiccups,but errors occur.
Now,if you choose to install and use both these programs,keep on reusing them and reusing them until they come up clean.I suggest you look through more threads and watch some YouTube videos on these programs so you can get a better understanding,as I'm not the best at explaining these things,and there are people better suited for it.
And like the the two mentioned above,use CCleaner,it will free up space by getting rid of duplicate documents and just stuff you don't need.Again,YouTube will help explain these things.
Now,I'm new to this myself,and I'm just telling you what I know,and hope you get help from someone with much more knowledge then me.But,in the meantime,I hope this helps,I'm just starting to realize how serious viruses and Malware can be,and I'm taking these measures,and hope you do as well.