WinAVI will convert from/to the formats you require exeng. You can try it free, think it's about £15 for full version.
http://www.dvd-guides.com/content/view/58/59/
Once files converted to DVD format you can use your Nero to burn to DVD. If you want to re-author then I use the excellent DVD Shrink (which is free)
http://www.dvdshrink.info/index.php
DVD Shrink also allows you to make copies of any DVD's you have, which is particularly useful as you can re-author it without all the "extras"/"trailers" etc. Obviously one must not infringe any copyright laws.
Good luck, Scally
ps. this may be better link for shrink
http://www.dvdshrink.org/where.html