This is entirely possible, although it may not actually be a virus, various 'malware' (Malicious software) exists that could be running on your PC or it could be that your current anti virus program has been compromised and is no longer detecting known virus's.

Initially a online virus scanner can be used to see if your current anti virus program is still operational.

Trend Micro Online Virus Scanner - homepage.

A free online Virus Scanner that you may wish to run if you suspect a Virus infection. This is an ICSA Certified Online Anti Virus Scanner.

Instructions: first Select "Scan Now. It's Free!", then again. Check "Yes, I accept the Terms of Use" and select "Launching HouseCall", then select "Starting HouseCall" (Java or Browser Plug-in) and wait. Then select "Next" (Scan complete computer for malware, grayware and vulnerabilities) and wait (this may take some time).

If no virus's are found then it is possible that another form of 'Malware' has entered onto your computer. There are various free Malware removal programs and a few are listed below.

Spyware Cleaning

The following free scanners will remove the majority of known Spyware. Windows Defender will be updated automatically via Windows Updates but Ad-aware and Spybot - Search and Destroy should be updated each week. CCleaner should be run before all the other scanners in order to save on scanning time.

Difficult to remove Spyware will require a more thorough scan in Safe Mode. The first time you run these scans it is highly recommended you download, install and update Ad-aware, Spybot Search and Destroy and Windows Defender, then run them all for the first time in Safe Mode (Press F8 during boot up).

CCleaner - Download - Homepage - is a freeware system optimization and privacy tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. But the best part is that it's fast (normally taking less than a second to run) and contains NO Spyware or Adware!

Microsoft Windows Defender Download - Home Page
Microsoft Windows Defender is another free program that helps protect your computer against Spyware and other unwanted software. It features Real-Time Protection, which is a monitoring system that will recommend actions against Spyware when it's detected.

Instructions - Select "Scan Options", and "Full Scan". difficult to remove Spyware may require a more thorough scan. You will need to reboot your computer into safe mode by pressing the F8 key down during boot up and selecting "Safe Mode" from the Windows Advanced Options menu. When in safe mode you can do another scan and then remove the remaining Spyware infections.

Ad-aware SE - Download - Home Page
This Application from Lavasoft scans and removes Spyware, Adware, hijackers and other malicious software.

Instructions - Install, Update and Run. Select "Start", "Perform Full System Scan", "Next" and Remove any Spyware items. In order to select all items for removal right-click on each of the found items and left-click "Select All". difficult to remove Spyware may require a more thorough scan. Reboot your computer into safe mode by pressing the F8 key down during boot up and selecting "Safe Mode" from the Windows Advanced Options menu. Once in safe mode do another Ad-aware scan and remove the remaining Spyware infections.

Spybot - Search and Destroy - Download - Home Page
This application scans and removes Spyware, Adware, hijackers and other malicious software.

Instructions - Install Spybot, select "Update", "Search For Updates", tick the box next to each update and select "Download Updates" and then select "Search and Destroy", "Check for problems" once scanning is complete choose "Fix selected problems". Difficult to remove Spyware may require a more thorough scan. Reboot your computer into safe mode by pressing the F8 key down during boot up and selecting "Safe Mode" from the Windows Advanced Options menu. In safe mode carry out another Spybot scan and remove any remaining Spyware infections.