I think that Norton AntiVirus is corrupting some files - has anyone heard of anything similar?

I'm using Windows 7 with encrypted files and files that I open and look at often seem to get corrupted shortly afterwards.  I know that they saved correctly because I can restore them from backup.

 

The only thing that I can see is that soemtimes I have a lot of disk activity when (as far as I can see) there should not be much.  Task Manager shows that AntiVirus is the main thing running and then I see that some recently touched files are now corrupted.  But there's no smoking gun - obviously AntiVirus runs to scan files and has to read the disk to do so.  And I can't tell exactly when the files are corrupted - the date stamp is not changed.

 

Has anyone heard of something similar?  This is driving me crazy.