There is always the possibility, given the rapidity with which malware is written and then spread through the internet, that you can become infected before the anti-virus company can write, test, and issue a definition. Your chances of this happening, especially if you keep your AV's definitions updated, are probably so slim that you should not be overly concerned. Remember, too, that any subsequent scan should find and quarantine the new virus.
There are many free on-line virus scanners available; the best of these are listed under the appropriate section of the BC Freeware Guide
These should be used periodically as a "double-check" to your own resident AV.
" F-Secure Online Virus Scanner is a free service. Use it to find out if your computer is infected and disinfect your computer if needed.
The size of the download package is about 22 megabytes including databases. "
I hope this helps,