While trying to install a program, it complained that something else was being installed and that Windows Installer was already running (msiexec.exe). Sure enough, there were two instances of the process with one of them using 25% CPU (quad core, so basically 100% of one core).
After searching online, I ran across this: http://support.microsoft.com/kb/2793634 titled "Windows Installer starts repeatedly after you install SQL Server 2012 SP1" which says:
"After you install SQL Server 2012 SP1 on a computer, the Windows Installer (Msiexec.exe) process is repeatedly started to repair certain assemblies." ... "When this issue occurs, you experience high CPU usage."
Has anyone encountered this or been able to fix it? I just thought it would be a good idea to have this question here with a solution for future reference, since this is a huge problem. It's impossible to install any software after installing SQL Server 2012 SP1.