Inability to Access Other Programs

[MrM21632]

I have an issue that has just come up - for some reason I was unable to access other open programs while I was working on something in another window. I had the following programs open:
- Firefox
- Windows Explorer with QTTabBar
- VLC Media Player
- Notepad
While I was in Notepad, I was unable to access any of the other windows. I was able to access Task Manager after closing the Notepad window, and restarting Windows Explorer seemed to fix the problem. Is there anything in particular that may cause this problem? Is this something that you guys have seen before, or could this just be a bug of sorts?

Update: This has occurred again, but now only Windows Explorer and Firefox were open. Does anyone have any ideas about what may be causing this?

 

[Tradesman1]

Have you run a malware scan as well as a virus scan?

 

[MrM21632]

Have you run a malware scan as well as a virus scan?

I have - it found nothing. I may run another in Safe Mode just to be safe, but I'm fairly certain it won't find anything. I tend to run a clean ship.