Can I use any other antivirus or Malwarebytes Pro or any other second opinion scanner along with Microsoft Security Essentials?

Well, other antiviruses and/or Malwarebytes Pro and/or any other paid (or trial) software provide Real Time Protection i.e. they work in background and scan your files and network traffic to prevent malwares (just like your antivirus). It is not recommended to have two or more Real Time Protection software running at the same time. They slow down your PC, slow down PC startup, causes system instability, increase PC resource usage, conflict with each other and sometimes even one stop other one from removing a threat, as a result, PC security is decreased (contradictory to what you wanted from two antiviruses).If you want a second opinion scanner then you can install these tools:

1. Malwarebytes(Free)
2. SUPERAntispyware(Free)
3. HitmanPro (Recommended by me over others)
4. AdwCleaner (Removes Adwares only)

Now question arises – What is the difference between these tools and the paid versions?
The main difference between these and paid ones is the Real Time Protection, these didn’t work in background. Free ones don’t have Real Time Protection (but they will ask you to buy their paid ones, never do that). These tools are considered as good second opinion scanner. If you think your PC is infected or your antivirus detected any malware and you want to be sure about its removal then you have to just open these tools, update them and full scan your PC.

People often complain one thing – I downloaded Free one but Paid one is installed!
It happens when you install their trial paid version (an available option with free software). See solution for this for different products below:
1. Malwarebytes
After you install it, you can see three check boxes, one for Launching it, one for Updating and one for enabling free trial version. Uncheck the one with free trial option (see red ellipse in image below) and click on Finish.

2. SUPERAntiSpyware

After installing SUPERAntiSpyware you will see this window. Click on Decline to use free version, not the trial one.