I found this article which describe how to find the install path of office:
http://support.microsoft.com/kb/234788/en-us
But in the article are only the GUIDs up to office 2007, what are the GUIDs for office 2010 or where can I find them?
I use these registry entries:
HKLM\SOFTWARE\Microsoft\Office\11.0\Word\InstallRoot\Path
HKLM\SOFTWARE\Microsoft\Office\12.0\Word\InstallRoot\Path
HKLM\SOFTWARE\Microsoft\Office\14.0\Word\InstallRoot\Path
If you have a MSI installer you can use RegLocator table (registry searches).