I went into the block library and found some excellent parts.  when I downloaded the parts, it appeared that some parts (such as the wheel) were assemblies but they had the .ipt extension.  This means that it's more difficult to dissect the drawing.  I also want to upload some of my own parts (i.e. a threadless headset) but I don't know how to convert an assembly into the type of .ipt which is required (zip files aren't allowed).
Thanks in advance for any advice.

Fabian;
Autodesk Inventor is a great tool for mechanical design and it works great for showing detailed views and animations of parts.  I don't know how well it would work for creating an animation of the whole bike possibly with terrain.  You could also look into 3D Studio Max or Miya.
If you decide to use Inventor though, I have some parts of a bike already completed.
http://www.flickr.com/photos/60961560@N00/sets/72157607894122095/
email me through www.tarfman.net if you want to talk about that.
Here's a tutorial on using Inventor for animation

http://www.youtube.com/watch?v=YhYYZ...eature=related
http://www.youtube.com/watch?v=YhYYZ...eature=related

What kind of help do you need?

As 3D Software Autodesk Inventor is the best choice. Or do you need somebody to draw it in Inventor?