Nomad_Soul: All attempts at downloading/patching have been done as an administrator.
Namur: By "done as an admnistrator" do you mean that you're on an admin account or that you're running the.exe with a right click "Run as admin" ?
I think you may have put me on the right track. I managed to get the game patched, but I took sort of a "shotgun approach" and I'm not 100% what ultimately did it. My initial install attempts were done via an admin account, but I did not use the run as admin command.
Here's how I finally managed to get patched in the hopes it will help someone out there:
First, I excluded both my install directory and my \My Documents\Witcher 2\ directories in my Antivirus/Security Suite just to ensure this wasn't causing issues. I had previously and temporarily disabled it, but perhaps this alone was not enough.
Next, I ran the "patch2_0_INT.exe" (the appropriate patch file for me, may vary for others) under an admin account with the run as admin command with the "protect my computer..." option unchecked. This attempt actually did produce another error, but read on.
Realizing that the patch2_0_INT.exe file basically unzips a file called patcher.exe into \My Documents\Witcher 2\Downloads, I then went there and used the run as admin command with the "protect my computer..." option unchecked on patcher.exe.
After all this, the patching process did successfully finish. I booted up the game, noticed the new options, and then loaded up a save. Everything is looking good.
Thanks for the replies folks. I'm glad to be up and running, and it's much appreciated!