Virus scanner is totally unneccessary for Pocket PC now. THere only two known virus for PocketPC and the chance of getting them is less than winning Aximsite contests.

Virus scanner only slows down your Pocket PC and consumes system resources. Also, virus scanner always messed up my Pocket PC since they mistaken LOTS of files as viruses.
Most PocketPC virus scanner, however, can detect desktop viruses. The only reason some people install virus scanner on their Pocket PC is to prevent transfering viruses to the desktop computer. For me, it doesn't matter at all because the virus scanner on my computer scan most files coming/going.