I'm experiencing the site locking or freezing a fair bit these days, it happens when I have a form open and I'm writing a PM or game post. The page may freeze for around 10 to 20 seconds. I'd say I encounter these freezes about 3 to 4 times a day, and not necessarily when my AV is running a system scan. I'm not necessarily blaming E; it could be me.
I logged two of these today: 12:15:16 pm for about 10 seconds and a 30 second freeze at 19:49:23. I'm on BST, +5 hours ahead of EST.
I'm using Firefox 3.5.9 and WinXP SP3.