Here is the text from the link I posted above in case the link becomes dead. Note that if you also want the Windows Installer to run from Safe Mode With Networking, you will have to paste a slightly different command into the command prompt as shown below:

Windows Installer will not work under Safe Mode, this means that programs cannot be installed or uninstalled in safe mode without giving a specific command using msiexec in command prompt.

To make Windows Installer work under safe mode, you need to create a registry entry for every type of safe mode you are logged in to.

1.Safe Mode.

Type this in a command prompt:

REG ADD "HKLM\SYSTEM\CurrentControlSet\Control\SafeBoot\Mi nimal\MSIServer" /VE /T REG_SZ /F /D "Service"

and then

net start msiserver

This will start the Windows Installer Service.

2.Safe Mode with Networking

REG ADD "HKLM\SYSTEM\CurrentControlSet\Control\SafeBoot\Ne twork\MSIServer" /VE /T REG_SZ /F /D "Service"

and followed by

net start msiserver

This will start the Windows Installer Service.