I saw this article at PC World:
http://www.pcworld.com/pcwtoday/article/0,1510,17593,00.html
Here's an excerpt:
On June 30, Symantec posted new definitions for Norton AntiVirus software. But when some customers received the automatic download through LiveUpdate, their systems stalled. Symantec says the problem only affected people running Windows 2000 on certain hardware configurations....
"We found that people who had Windows 2000 under a special hardware configuration using Promise ATA ultra-66 hard drive experienced a slowdown or halt during boot sector scanning," says Vincent Weafer, director of Symantec's AntiVirus Research Center. "But most people wouldn't have this configuration."
This may be old news, but I just saw it.