Help debug why my Garmin GPS software and receivers stopped talking?

Please, if you are into this sort of thing, help me work out what has gone wrong between my Garmin MapSource software and Garmin receivers. The software is no longer able to detect or communicate with Garmin GPS receivers, though it worked fine for years.

The problem is not the hardware connection. I also have G7toWin and Fugawi, and both of these still communicate just fine in both directions. I am also able to communicate in text mode with a terminal emulator program. In all these test cases and in my past use of MapSource I am using the same Garmin RS232 serial cable plugged into COM1 on the same PC. I now use several GPS 60 and one GPSMAP 76, and the several I have tried still work with these other programs. I do know to set the receivers to Garmin mode for connecting to MapSource. I have checked for processes named active process named Activesync, Hotsync, BBDVMonitor, Besmonitor, Intercept, LVCOMM, Tablet, UPS, WCESComm, WCESMgr, WeatherPak, x10net, or ZUNE per Garmin’s web site tech support solution for lost serial link, and there are no such processes.

I have also tried using a USB connection, and MapSource also fails to detect my GPS 60 receivers using the USB connection. I never tried this before and do not know if it would have worked previously.

There are no known changes in my PC that coincide with the beginning of this problem, except that I have installed Apple iTunes. If iTunes and MapSource were incompatible in some way, it would be consistent with what I have seen. I have tried reinstalling MapSource and iTunes, and also disabling my antivirus protection, but the problem remains.

My current MapSource program is the Trip and Waypoint Manager V6.0. “USB Driver Versions…” reports that no USB drivers are installed. I was accustomed to using the RS232 serial connection and would be happy to continue doing so, especially considering that the GPSMAP 76 does not support USB.

My PC is a Dell Dimension 4600 running Win XP Pro V5.1, with Service Pack 3.

I have tried submitting this prolblem to Garmin tech support using the form on their web site, but when I click “Submit”, I get what looks like a page on their site saying “Internet Explorer is unable to display this page”. I don’t know if this means the submission worked or didn’t.