Hi again Liorlankri,
You have discovered a vexing issue. We have had the same issue before for a previous project. Bottom line: full trust is not worth doing because of the deployment hassle around .NET framework, certificates, etc. In addition, installing a form using an MSI (that you have created with the regform tools) removes the ability to have it interact with a website, since it removes the URL from the solution and makes it purely URN-based.
You can make Full Trust work .... here's how:
For the certificate, you'll want to either a) install Certificate Authority service on your Intranet server, or b) buy a third-party digi-cert (for example, from Verisign). The third party ones are going to cost a lot (~$500). The Cert Authority requires that your users go through a multi-step import process.
If you are going to sign the form, then use the Send as E-mail option to send the signed xsn to your users. If you use regform to create an MSI, there's a /FT option to make it full trust by default. That's probably a better solution. You will need to save the template or publish it locally in order to run the command-line regform tool on it. After you create the MSI you will have to send this in e-mail (probably also add an "_" (underbar) to the msi extension so that Outlook doesn't block it.
This is complicated. There is no silver bullet here, but you can make it work. We have.
Good luck,
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra