On Windows Vista and Windows 7, with the default settings, we can’t install unsigned third party drivers. We need to enable test signing to install such drivers. We can enable this from command prompt using the below command.
bcdedit /set testsigning on
Note that you can run above command only from elevated administrator command prompt. Otherwise you will get error as shown below.
c:\>bcdedit /set testsigning on The boot configuration data store could not be opened. Access is denied.
See How to open elevated command prompt
After running the above command, you can verify that test signing is enabled by looking at the boot loader settings which can be retrieved by running ‘bcdedit‘ command.
C:\>bcdedit
Windows Boot Manager
--------------------
identifier {bootmgr}
device partition=\Device\HarddiskVolume2
description Windows Boot Manager
locale en-US
inherit {globalsettings}
default {current}
resumeobject {8d354cb4-d349-11e0-be61-5cff35092b52}
displayorder {current}
toolsdisplayorder {memdiag}
timeout 30
Windows Boot Loader
-------------------
identifier {current}
device partition=C:
path \Windows\system32\winload.exe
description Windows 7
locale en-US
inherit {bootloadersettings}
recoverysequence {8d354cb6-d349-11e0-be61-5cff35092b52}
recoveryenabled Yes
testsigning Yes
osdevice partition=C:
systemroot \Windows
resumeobject {8d354cb4-d349-11e0-be61-5cff35092b52}
nx OptIn
C:\>
To disable test signing you can run the below command
bcdedit /set testsigning off

run cmd as admin and it should let you sign it.
Thanks for leaving comments..It's already added in the above post that cmd should be run from administrator account..(check the line above the command)
Why don’t you add the obvious info that this operation cannot be performed on a 64 bit version of Windows 7?
The command works on 64 bit windows too.
I am getting this error
As mentioned earlier by Doug this will not work on 64bit Windows (tried it on Win7 – command can be executed but will not allow non signed drivers to load)
See:
https://docs.microsoft.com/en-us/windows-hardware/drivers/install/the-testsigning-boot-configuration-option