DVDs now mostly unreadable under OS X 10.6.4 but it's not hardware

I have a problem with getting this message when I insert DVDs into my Mac Pro

"The disc you inserted was not readable by this computer."

This occurs when I try most DVDs, such as a DVD containing scanned images I just got back from a scanning service. It occurs when I insert any DVD I have previously burned on this machine. Right now, about the only DVD I can open on my machine is the Mac OS X Snow Leopard installation disc.

When I boot up my Mac under Boot Camp in Windows XP SP3, I can read any DVD in my Mac under Windows. So I am presuming the problem is not bad DVD hardware.

Here's what I have tried:

1. Booting up in Safe Mode: I can only read the Snow Leopard DVD. Other DVDs spin up but then the drive stops, although I do not get an error message

2. Tried doing Repair Disc Permissions twice on my Mac boot drive. A bunch of things were fixed but not the DVD problem.

3. The discs I created previously on my Mac are easily read on a different Windows Vista laptop, so I think there is nothing wrong with the discs.

4. Discs created on my other Windows laptop are readable on my Mac when it's booted up in Windows but not when it's booted up in Snow Leopard.

Here's one other oddity:

I used TrueCrypt to keep an encrypted file container on my boot drive. I DO NOT encrypt the entire drive. It's just basically an encrypted filefolder. Just a few days ago, TrueCrypt has been unable to mount these encrypted files on this machine under Mac OS X. However, when I boot up under Windows, TrueCrypt, which has both OS versions, can read the file just fine.

I think I am going to reinstall Snow Leopard, but I thought I would ask here first.

Thanks, all.