This option can be explored if you already have paid for an antivirus and you want to keep it, and you do not want to get into complications of installing two antiviruses on the same computer.  There are several good antivirus sites which provide online virus scanning.

Some sites only tell you what is infected on your computer but do not remove it. Whereas some sites generously detect and remove the viruses found.  I am going to list such sites which delect and remove. And of course they all offer this service for free. The prerequisite for this scan is a functioning browser and an internet connection.
1) BitDefender Free Online Virus Scan. Click to visit site.
2) TrendMicro HouseCall : Click to visit site.
3) Eset Online Scanner : Click to visit site