The title says it all.
Here's what I ruled out:
1) Bad media: I have the error with TDK DVD+R, Memorex DVD+R and Philips CD-Rs. These are name-brand DVDs and they burn on another PC. (Oddly, Windata DVD-Rs work fine).
2) Bad DVD Drive: Same error with another DVD drive. I was able to burn with the same DVD-drive connected to another PC.
3) I also replaced the PC's power source and still got the error. When I put the DVD-drive in an external enclosure (with separate power) I still get the error.
My guess, is that something on my PC is interfering with the burning process. What could it be? (I shutdown all programs including Explorer before burning)
A possible clue: Before this error, I would get no error during burning the DVDs but I couldn't play the DVD on the same PC. I was able to play them on other PCs.