in answer to your questions....
a program written in vb6 will not compile under
vb.net because the two development tools are based upon completely different engines. in vb6, you were building applications that utlitized the "runtime". in
vb.net, you are building applications based upon the .net framework. each has its own compiler.
there is a utility within the vs.net package, which will go through a vb6 project and "upgrade" it to
vb.net. basically, in the
vb.net ide, open your vb6 project & the upgrade wizard will start. according to the size of your vb6 application, the time taken by the wizard will vary.
the wizard will make a number of changes and open it within
vb.net. your todo list will have a number of items, which require your "human" attention. as with most other "upgrade wizards" and code generators, you are trying to circumvent alot of manual work by having an application do alot of the work. how well it succeeds is really relative. is success that you didnt have to do everything to update the code; you might consider it a success. is success that the wizard took all of your vb6 code and "magically" changed it to valid, well-written
vb.net code; you will probably be disappointed.
as for the "differences" in vb6 versus
vb.net - i'd advise you to hit the msdn site, gotdotnet.com, or some of the other sites that give more indepth coverage to the tools. there is noone who can really answer that question in a listserv response with any quality.