Go into safe mode with networking, grab a copy of malware bytes, install, update and run.
If its a big one (rootkit, trojan.downloader etc) best option would be to reformat and start again. Malware bytes should tell you what it is, and attempt to get rid of it.
There are a raft of bogus av/malware killers/general rubbish doing the rounds here - which is good for me, but not so good for my customers.
Wurzel
