I have experienced this same problem with several of my clients. All the clients are using Office 2010 (32 bit) with Windows 7 (64bit) and in an SBS 2008 environment. They are using different antivirus and in some cases, I have even experienced the problem while AV is disabled. Here is the problem:
The user opens up a Word or Excel document and a dialog box pops up saying "downloading" with the file name. The address bar starts to crawl from left to right and eventually, after at least 5 minutes, the file opens. In some cases, the file doesn't open at all. Either a system reboot or some other major closing of applications resolves the problem. I have even seen this problem happen to ME although not very often. One thing I have noticed is that most if not all of the files have been xls or doc files and not their 2007-2010 equivalents. This should not matter but that is something I noticed. I am quite certain it is related to Windows 7.
This is a major problem for me and in fact, I may lose a customer because of it. We have changed PCs, changed user profiles, changed cabling, disabled AV, checked logs (nothing much there), ran additional logging, and still no resolution.
Please help if you have any thoughts. I have had 3 different technicians look at this so this is not a simple problem.