When you try to install RC4 over RC3, the previous version is uninstalled.
IF you aleady have a prevoous uninstall file in %TEMP% then you receive a .NET exception and the installer crash.

h4. Steps to Reproduce
Install RC2
Upgrade to RC3 (first time uninstall-mp.exe is copied in %TEMP%)
Upgrade to RC4 (uninstall-mp.exe is already found -> .NET exception)

h4. Additional Information
http://forum.team-mediaportal.com/testers-lounge-68/xpsp3-install-error-49259/

Reported by Paranoid Delusion, Psycho Reptile and a community user.