I've been playing New Vegas for the past 2 weeks and haven't run into any errors until now.

My problem:

When trying to launch New Vegas (through steam, or otherwise), as expected I see a "Preparing to launch Fallout New Vegas" steam window. Only, this just continues to "prepare" for as long as I decide to leave it there, constantly opening, closing and re-opening the "Preparing" window. I've been running this game perfectly and without fault for the last two weeks until now. My guess would be that this may be related to the few mods I had installed, though they weren't causing any problem until now. I uninstalled them, hoping for success, to no avail. As a last resort, I completely uninstalled New Vegas from my computer, checked the steamapps folder for Fallout New Vegas and removed the few small files that were left over from mods. Reinstalled the game, downloaded from steam what it asked of me. And yet, no success.

Tl:Dr - Will not launch, stuck in "preparing to launch" screen.

Any help would be greatly appreciated.

Did you delete the fallout 3 nv directory when you uninstalled?
YOu might want to try copying your saves in the documents folder and deleting the folder.

latest run-time
vc2008 run-time sp1:
http://www.microsoft.com/downloads/en/details.aspx?familyid=A5C84275-3B97-4AB7-A40D-3802B2AF5FC2&displaylang=en

Also try installing the latest directX.