Converting media files from one format to another is a pain in the rear often times. Lifehacker has a good article on this.
Check out the whole thing here.
Incidentally, I highly recommend Ulead Video Studio for dealing with HD.
Yes converting media files can be a pain in the back side. I've got a copy of Ulead around here somewhere, but have never used it. I'll have to give it a try.
I've been using Sony Vegas lately, seems to do the job nicely.
I picked up a copy of Cyberlink PowerDirector for my HD editing. If I just want to convert AVCHD to MPEG, Ulead is what I use.
I have Ulead Video Studio, Ulead DVD Factory/with Ulead Cool 3D, and Cyberlink DVD Solution, which is a suite of apps including Power Producer, Power Director, Power DVD and MediaShow. For a home user, you can pretty much do anything video/DVD related with those in your arsenal.