Thank yo so much for your time looking into this :D

I've been thinking the whole day what I did to have caused it. I was quick
to give SP3 the fault, and realized that this occurred about 2 days after
installing it. So SP3 is not the problem.

The only thing I can think of was I used a tweak program (PC Health
Optimizer) to hide a drive in My Computer called G:. This drive was created
by installing a Canon MP520 Printer, and seemed like a useless entry for me.

So I think the problem started turning the printer on/off and it could not
display the G: drive.

I only tweaked the hiding drive, and removing "Short-Cut to..." in the tweak
program.

So when I right-click the Start Button and click Explorer/Open, I don't get
the '(null)' error. It's only when I use WinKey+E, right-click My Computer
and clicking Explore. Maybe It's pointing to something in the registry that
My Computer should show, but can't. I'm gonna do some system restores, to
see if it fixes it :p