I think this is due to my use of a high resolution icon. I added 256x256 and 512x512 variants of the icon for the companion app, but XP is so old that it can't handle the icon. I guess I'll have to get rid of the higher resolution variants of the icon for the XP users. You can download 2.6.8 here:
www.zubersoft.com/mobilesheets/archive/2.6.8/MobileSheetsProCompanion.exe
www.zubersoft.com/mobilesheets/archive/2.6.8/MobileSheetsProCompanion.exe