Yeah, I'm at a full day now where the computers w/o Norton have been fine. It's fixed.
Thanks to all for your assistance with this issue, including DADanielEJ33 for posting it here, ZigZag3143 for suggesting it was Norton early on, and Microfiche, tnaant, CSVJS, DynaEan, KentKuhler, Superior82, and anyone I missed for taking the time to chime in and mention that they had Norton too. Also kudos to XP ROCKS! for pointing out the SYMEFASI64.SYS connection to Norton.
This problem was... frustrating. It's been ages since I've been on a problem where Google was such a wasteland. Glad to have somehow found this thread. Also, that "Norton causing BSOD" thread Microfiche linked to looks interesting, especially for those who don't plan on hitting the problem with the "uninstall Norton" hammer.
Thanks all!