Since it's working for some users but not others, you've probably already figured out that it's most likely a local machine config issue.
I'm trying to understand at exactly which point does the edit this task window not popup?
InfoPath can be picky on the local config. you may need to fully uninstall Office 2007 and Office 2003 on those non-working machiens then fully reinstall Office 2007 to see if you can get it working. Another option is to try a detect-and-repair on the Office 2007 installation.
Shiraz Cupala / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge