Data Execution Prevention Closing Internet Explorer and Google Chrome

Hello, as you can see I'm new to this forum but not new to forums in general.  I am not sure if this is the correct area to post this but I did not see an area specifically for virus threats.  I may be overly suspicious, but I believe my computer may have a virus that Symantec is not picking up.  It seems like it only effects my internet browsers though. To give a little background:

 

OS: Vista Home Prem 32-bit (current on updates)

Norton: Internet Security 2009; Antivirus 2009 (both up-to-date)

Internet Explorer 8 and Google Chrome 3.0.195.38

No other antivirus or browsers installed

Ask if you need any other info on my setup 

 

Issue:  Seemingly randomly, both IE8 (opened normally and "Without Add-ons") and Chrome will shut down with an error from DEP saying "Data Execution Prevention - Microsoft Windows. To help protect your computer, Windows has closed this program."  I thought it was weird that IE8 just started closing one day and then I installed Chrome and almost immediately it started closing because of DEP.  They shut down at random and I am unable to duplicate it because it does not happen specifically on one site or by clicking a specific link.  DEP does not allow me to add either browser .exe to the DEP "exception" section under "Advanced System Settings>Advanced>Performance-Settings>DEP".  The DEP does not effect any other program I have ever run on this computer so I have to assume there could be a virus somewhere that Symantec is not picking up, has not added to their Definitions yet, or one of the sites I visit regularly has an active virus.  Sites I visit regularly include Facebook, MySpace, Google (and branches of Google), Hotmail, Gmail and USAA Banking.  I do not visit "dirty" sites and do not click on popups nor do I fall for the common "Your computer is being attacked.  Click here now to start a virus scan" type popups or pages.  I have noticed this complaint on both the Google Chrome and Microsoft forums but neither has been solved.  After using the Search I didn't find much and I was just curious if Symantec or any Symantec product users have experienced the same or knew how to fix this?  Thank you all so much!