We have struggled through this exact same problem.  It's not the "File 
Exists" part of the check that is failing, it's the "File Version" part 
that doesn't work.

Since we only have experienced a few of these devices in our student 
dorms, we have a staff member manually create a "Dummy Keyfile" after 
verifying that the user has the proper version of symantec installed and 
running.  So our rule actually has an "OR" statement.  Fileversion of 
RTVScan.exe = 10.x.xxx OR "Dummy Keyfile" exists.

Obviously this isn't the preferred method, but that is where we are stuck 
for now.  I'd be more than happy to hear other suggestions ;)