I am having the same problem here. My internet connection speed was fine until Avast updated to version 5.0.545, then connection speed slowed down dramatically, slow page loading, slow tab opening and pages locking up.
Before I isolated Avast as the culprit, I disabled ZoneAlarm and Malware AntiMalware, no difference. Talked to IT guy at work, told me to disable the Behaviour Shield on Avast. I tried that and my connection speed is back to normal, no more slow loading of pages or tabs.
Saw many topics on similar issues and was not sure which topic to choose.
my system stat
Gateway 504 GR
P4, HT 3.00GHz
1.50 GB Ram
XP Home SP3 Version 2002
IE8
Avast 5.0.545
Malware AntiMalware 1.46
ZoneAlarm 9.2.044.000