Cross-posting some of my findings I also posted in http://social.technet.microsoft.com/Forums/en-US/FCSNext/thread/2b853e6a-b05f-467d-bd3e-31f0cdf5830e
I’ve been having the same problem with several servers. I have found that uninstalling FCS can sometimes be a pain.
If you open your REGEDIT and find the HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\UserData\S-1-5-18\Products folder and here search for ForeFront you will find the 2 installations from ForeFront.
NOTE: This was done on 2008 servers, on 2003 the Registration path is: HKEY_Classes_root\Installer\Products
What I found on some machines is that it’s worth trying to force an uninstall using a command prompt (Run as Administrator) and then run the UninstallSring key found in these Product listings manually but using /X instead of /I.
If that doesn’t remove it completely (For me in some cases the mp_ambits.msi was installed using an old server that was bugging out due to it forcing the installation of x86 installers on x64 machines) remove these two keys manually, then reinstall the proper verison of FCS and then see if FEP can remove it via te installer or if you still have to then uninstall the normal old fashioned way by hand or via MSIEXEC.exe /X.
After removing the FCS and before installing FEP you may also want to remove the following registry keys