I've had two bad experiences upgrading an existing install of Replay Media Catcher 4.x. My first was from 4.0.5 (I think) to 4.0.9. The second going from 4.0.9 to 4.0.10. I'm running the 64-bit version on Windows 7 Ultimate x64. Here is the scenario, exactly the same for both update attempts:

1. RMC is not running
2. Start installer and say "Yes" to the UAC prompt.
3. Progress bar reaches approx 10% then stops. After 10 minutes it's still stopped. All networking is lost. Sysinternals Process Explorer shows 5 msiexec processes. The lowest level in the process tree is "snetcfg.exe". I'm able to kill all but the lowest level msiexec processes. Attempting to kill snetcfg.exe gives "Access Denied". I am running Process Explorer in elevated mode so I guess the system is holding on to this process.
4. Reboot the PC.
5. After the reboot, my network is working (phew!).
6. RMC 4.09 is still listed in Uninstall Programs, so I uninstall it.
7. Rerun the 4.0.10 installer, and this time it completes with no problems.

I note that the Replay AV installer prompts with advice to uninstall and reboot. My experience is that the same advice should be given by the current RMC installers. I can't see anyone else posting this issue so perhaps it's an x64 issue only.