I'm running Act! V9.01.162 Hotfix 1 and I have the exact same problem. All of my ACT windows and dialogs open find EXCEPT the Opportunity window.
It appears to open, but doesn't display on screen because the base ACT window locks in typical dialog-mode behaviour where the top window is the only accessible one. When I press ESCape, I'm guessing the Opportunity window closes because control is returned to ACT.
I had been using the ACT! Palm app with no problem and even uninstalled it and rebooted after reading this, but it made no difference. I have no specific idea when the problem started other than within the last 2 months because I use the Opportunity window frequently.
I even re-installed ACT from the CD. No change.
I'm running XP SP2 with 1 GB RAM, Office 2003 latest packs, and Avast Antivirus - none of that has changed in the last couple of years.
Any suggestions or tips would be greatly appreciated because if I can't fix this, I'm going to have to abandon it - I need that opportunity feature.
-steve