backup important data (documents, pictures, etc) to external drive, and then format.
Most times, the files that are infected by an outsider, are too much well hidden to be found, even for an antivirus software.
The thing is, those files are encripted, and if the encription was done by him, it is called a "FUD" (Fully UnDetected), hence, not detectable (lol going round and round).
Also, the process and services being run, are also usually with a name that is very easily mistaken as a OS service/process, so it is hard for you to find.
So sugestion 1 (as said before):
- backup files that are .doc, .xls (excel), pictures (.png, .jpg, etcccc). You get the point. Files you are not sure that are YOURS, just forget those.
- format everything.
If you still want antivirus software:
- Malwarebytes Anti-Malware (www.malwarebytes.org)
- Comodo Anti-Virus