Thanks for all the responses.
@ Audrey: The online help resources and technical support for the Xilisoft software is very limited. I went through it and couldn't find an answer. But there was a link to contact technical support by email, so I may give it a try. I did a Google search on the problem a few days ago and found lots of people having similar problems when ripping DVDs (using various ripping software), but few answers, at least few answers that a layman like me could understand.
@Trombone AL: I was thinking about doing that. Right now I can decouple the audio and video tracks in the .mp4 file using Quicktime Player but I haven't found a way to recombine them after introducing a time delay for the video track. The problem I am facing is that that .mp4 movies are not well supported by Microsoft and I can't open them in Movie Maker or even Media Player.
@ CCdaCE: I too thought that perhaps CPU load had something to do with it, so last night I restarted my computer and closed all applications and I ripped a DVD during the night. The result was terrible. On the other hand, last week I got some good results ripping the DVD while I was using the computer for other tasks and CPU-hog programs like Quicken and iTunes were running in the background, no problem. So CPU overload does not appear to be the problem.
@IndependentlyPoor: I already use Handbrake, but it takes seemingly forever to rip a DVD and that's why I wanted to try a different piece of software. The results with Handbrake are usually pretty good, no audio delay. I have noticed some image distortion with Handbrake (aliasing), but I may just have to suck it up and go back to it.
@RonBoyd: I will definitely give AfterDawn a try. It's the kind of self-help resources I love.
@easysurfer: I have one of those VHS/DVD combo thingy. Originally the idea was to convert my VHS movies to DVDs and then to .mp4 at a later date. Perhaps the VHS tapes are too old and perhaps too much quality is lost during the conversion process, I'm sot sure, but the results have been disappointing especially given the amount of labor involved. Or it could be that I have become so spoiled watching HD movies on a large screen TV that I have forgotten what it is to watch a VHS movie on a small CRT TV. I guess I wasn't so picky back then...