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.
I ran the very first suggested hotfix and it seems to have fixed the problem: http://www.microsoft.com/en-us/download/details.aspx?id=36215
I didn't have to register to get an email or any of that other annoying stuff that some hotfixes require. I just downloaded that and ran it. It performed some "stop check" first and suggested I should shut down MSIEXEC manually to "avoid restarting the system", but the Services manager wouldn't let me stop it, so I just ignored it and let it keep running anyway.
There were a few setup steps, and then after about a half hour of installation it finished and didn't require a restart. I was able to immediately install other apps. Problem solved for now.