Very, very interesting DavidR. I normally use an Explorer substitute called Directory Opus but I've checked with Win Explorer and they both behave the same on my system 'Scan <obj name>' for single selections and 'Scan selected area for viruses' for multiple selections.
I find the text for multiple selections more informative because includes the word 'virus', qualifying the scan as a virus scan. Howver it is not ideal. A better phrase would be 'Scan selection for viruses'. Ultimately, I think the preferred solution is to let the user choose if they are so inclined, hence my suggestion for a new option.
BTW, I'm using XP/SP3.