I ran into this a few months ago. But danged if I can remember what I finally did to resolve the issue.
I know I went to the Microsoft site and downloaded IE6 again ... and it took a couple times reinstalling it. Also did a few Scandisk's and Defrag's ...
All I remember for sure is cursing Microsoft, Bill Gates and Windows an awful lot ... but I don't think that actually helped resolve the issue though
Oh yeah ... and I ended up uninstalling ZoneAlarm Pro & EZ AntiVirus. I went with the free AVG antivirus and am using XP's built in firewall.
Anyway ... wish I could remember which specific thing actually fixed it. Sorry