You can use the registry to find the GUID of a piece of software for use with uninstalling with MSIExec.
In Regedit go to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\
Note: On a 64-bit computer you will need to check both the key above and the following key:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall
Find the program you are looking for and look at the uninstall string