Do NOT upgrade to SP1 unless you love pain. Way too many bugs and going back is almost impossible.

On WinNT4 SP6a, since going from IE6 to IE6 SP1:
After visiting certain (keeps varying) websites and then exiting IE, the iexplore instance remains stuck in the TaskManager.
Interestingly, it is not actually hung, as it will disappear from TM by itself after about 5 minutes.
I can also create more than one "stuck" instance of iexplore in TM, which will also eventually disappear.
Finally, trying to reboot with 1 or more "stuck" instances in TM will bring up the non-responding instance for manual clearing.
Time for another Microsoft patch, admittedly not "Critical", but very, very annoying.