As a second opinion, you could use:
- ESET Online Scanner (Download using non-IE browser)
http://www.eset.com/us/online-scanner/
- HitmanPro (Free, if you don't activate license)
- VirusTotal Uploader (Context menu to submit to VT)
http://malwaretips.com/Thread-Phrozen-VirusTotal-Uploader-3-0
http://malwaretips.com/Thread-VirusTotal-for-Desktop-Browsers-and-Mobile-devices
- You can install Avira Free without the Guard (real-time), but I don't suggest it.
If you haven't experienced any problems, then I can guarantee you'll experience problems once you install another AV, even if it's a "companion AV". etc. Have fun!
Although I recommend being more careful what you download, install or browse. If you use Chrome or IE, both uses light sandboxing techniques, more secure than Firefox. Remember to update your software and OS, as well as uninstalling any software you don't need (ie. Java or Silverlight). Use an AdBlocker on your web browser, to filter out the unwanted. Visit your trusted sites and bookmark them. And the list goes on...
Anyway, why do you need another antivirus?