CAS policy. Migrating from VB2005 to VB2010

Trying to migrate from 2005 to 2010 and having all kinds of issues. All my apps give me the CAS error and I am stumped. I can't really understand the things i've read about this.

Also having issues with NET 4.0. Can I install NET 3.5?

Any help will be appreciated. IS there some post about a migrating workflow.