Garry's Mod will launch from Steam, but then my entire screen flashes B&W before I have to use Ctrl + Alt + Delete...

Here are my specs:
Processor - Intel® Core™2 Quad CPU Q8400 @ 2.66GHz 2.67GHz
RAM - 6144 MB (6 Gigs)
Graphics Card - NVIDIA GeForce GT 220
OS - Windows 7 Home Premium 64-Bit (6.1, Build 7601)

Extra:
Monitor - HP 2009 Series Wide LCD Monitor

Problem:
So, I woke up today and found out that my GMod wouldn’t work. I’d open it from Steam and it would show the “Preparing to launch Garry’s Mod…” dialog box. As soon as that would close, my screen would go to black (as if the game was starting up). But then, it would just flash from black to my desktop over and over and over again!
I tried cleaning my GMod by deleting the garrysmod/garrysmod folder, but it did the same thing. When I checked, the new folder was there.
So, I uninstalled and reinstalled. Same problem.
I tried to update my graphics card, my processor, even checked for upgrades on my monitor.
They were all updated.

The only way to stop the flashing is to hit Ctrl + Alt + Delete and completely shut down GMod. I tried running other gamesm (Portal, Star Wars: KOTOR…), but they don’t do this. It’s just GMod.

Anyone know how to fix this? Biggest pain in the ***. I had some pretty good ideas for L4D screenshots. :confused:

PS: Sorry if I left something out. This is my first post on FacePunch.

EDIT: I’m hoping not to have to reinstall Steam. It took four days to download my Orange Box content. It’s a huge pain, because that slows down the internet at my house and my mom goes to online college.

It worked before, right?
Did it update at all?

It worked perfectly. Smooth, crisp…
I don’t know what changed. And there was no update.

Can anyone help? I’m still having this problem.
I also tried setting the launch options. -dxlevel 90… -dxlevel 80… -fullscreen
Nothing works.