I have the same, here goes some more details:
1. I'm using Windows XP SP2 with latest updates
2. avast! 4.8.1335
3. The problem clearly reproducible by this scenario:
a. starting up windows
b. with only Standard Shield _ON_ (others initially terminated), open windows explorer and right click on any relatively large folder.
c. reproduced - explorer.exe hung, the only way is to call task man, terminate explorer process and then start it over. After that right click on folder works just fine, no freezing.
d. if Standard Shield is initially disabled after system reboot, then the problem is not occurring - right click on folder is NOT freezing.
e. if Standard Shield is initially disabled after system reboot, but then after a while you enable it and right click on folder, then again the problem occurs
f. if explorer is in freezing state, terminating avast Standard Shield from GUI doesn't bring explorer back to live.
The freezing also impact overall system performance even on my Q9400 quad core.
Conclusion - certainly this is critical bug, which requires close attention of developers, if still haven't found. The only workaround at the moment every time system boots up to restart explorer.exe which is not bad.
Let me know if I can do anything to help trace this.
with hangrep tool I'm not sure which process to dump - is it helpful to see explorer.exe dump?