After talking to knowledgeable people on the digital SSTV group, I have decided that the "virus" detected by Norton is a legitimate file distributed with the program.
The confusion seems to be stemming from Norton's detection of a file called loop.zip, which contains a Windows library file called loop.dll. Loop.dll has been around for a few years, and it contains routines that are sometimes used with SDRs. However, another file named loop.exe is associated with several known Trojans.
This leaves only the odd question of why loop.zip would be recreated whenever EasyPal ran. After I completely uninstalled EasyPal, this behavior stopped, so there's no evidence that the system is doing it.
I have not tested the latest version of EasyPal, which is only a few weeks old, to see whether it, too stops this behavior. However, I now once again consider it excellent software.
Apologies to the programmers of EasyPal, who have really done a nice job bringing hams a more reliable way to use a complex mode.
Other discussions have centered on the widely known fact that there are probably better anti-virus packages on the market than Norton. While my version is better than the previous two or so, it's still pretty bloated and prone to causing issues such as this one with EasyPal.
Sorry about that.