Uhm, this problem happens with and without a router, so that option's void. As for the sharing, it happens on both Win XP and Win 7 Fresh installs (Admin sharing or otherwise). Would this by any change be my Ethernet card?
Do the XP machines freeze by chance when they try and access each other? Or is it just when they try and access the Win7 machine or the Win7 machine tried to access the XP machine?