It might happen that after an upgrade/installation of a SEP Client on an infected computer, the computer can no longer boot in normal mode but can boot in Safe Mode or Safe Mode with Networking.
You might then use the SERT to clean up the computer but if it fails, you might try the solution below.
1. Boot into Safe Mode,
2. Create the following registry key:
(If you also want it available in Safe Mode w/ Networking, add the "msiserver" key under the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SafeBoot\Network\ key found here as well)
3. Edit the "(Default)" value in the new "msiserver" registry key, and enter "Service" in the "Value Data" box. Click OK, and close the registry editor.
4. Reboot back into Safe Mode, and the Windows Installer service should now run and allow you to add or remove programs in Safe Mode.