Question about DVDs and video clips

Who here is knowledgeable about recording video clips to DVD?

I'm trying to use Sonic MyDVD to write various video files (.avi, .mpg, .wmv, .asf, etc.) as a DVD and I'm running into a problem. When I try to add certain video clips, I get an error message saying that they're in PAL format, and it won't add them unless all the clips are in PAL. Apparently, some of my video clips are NTSC and some are PAL format (but all are readable on my computer). I always thought a .wmv file was a .wmv file, but that seems not to be the case.

Is there software out there that will reformat a PAL-formatted file to NTSC? How do I get around this?