Wow, things like this are so frustrating. Here's what I do in situations like this. It's gonna sound simple but sometimes if you can't solve something you have to step back and start looking at details. Compare the PCs that do work to one that doesn't. Make notes on what's different about the one that does not work compared to the ones that are. For instance, on the one that's not working correctly this certain software is installed or this user is setup differently than the users of the systems that do work, etc. This puts a list in front of you so then you can ask yourself if there's anything on the list that would cause this problem. Also, have you checked the application event log to see if there are any clues there? And as an afterthought, on the PC that doesn't work, uninstall the software then run a program called msicleanup (I'll shoot you a link if you can't find it) and then install the software again and see if that helps. Good luck!