A virus is malicious software or a malicious program which hides in other program and execute when you open this program. A virus is inactive until you execute an infected program.
Technology has been changed and changing rapidly, different viruses has an ability to destroy your anti-virus software so in this case a online virus scanner probably a good choice to use.
Here are the list of some more common anti-virus software vendors online scanner web link. If your anti-virus do not work properly or if you have any doubt that your computer has been infected by a virus so scan your computer online.