|Unfortunately we have no way to know the depth of knowledge of posters, and the proposed use of System Mechanic in connection with msiexec.exe was not mentioned in your original post.|
MsiExec.exe if in "C:\Windows\System32" is a valid MS system file (Windows Installer), which is in use. You might also find valid copies of it in C:\Windows (service pack areas). The only proper uninstall is to tackle Windows Installer itself, which I would not advise.
System Mechanic is the sort of program best avoided because it can take out the wrong things and cause issues - I've seen such reports on these forums. If you wish to risk using Registry Cleaners then CCleaner Lite (free) is one of the safest, although, like the rest, it is not usually necessary and makes little difference to computer performance.
Always pop back and let us know the outcome - thanks