Hello Claire,
I will make a couple of comments here that will maybe help you out:
1. FullTrust is not always needed for IP2003 solutions with code. It depends on what your code is - if you do not use anything that requires full trust (such as writing to disk) you can use domain trust instead. Domain trust does not require an MSI installer.
2. If you do need Full Trust, your other option is to digitally sign the form. You can use self-sign certificate or purchase one. This will require you to spend some time investigating this option.
3. You can develop InfoPath 2003 solutions in Visual Studio 2005 if you have InfoPath 2007 and VSTO 2005 SE installed. You cannot develop with VS2005 and IP2003 unless you have the toolkit which is not available for a free download. This is a good overview of what's going on: http://www.infopathdev.com/forums/p/5661/21211.aspx
4. You can install VS 2003 and VS 2005 side-by-side without any harm. If you just need VS2003 for RegForm, this is probably your solution.
Let me know if any of these tips help. Also, feel free to post to our French forum if you prefer, I speak French so I could help you out en Français if you prefer.
David Airapetyan / Software Services ArchitectQdabra® Software / Streamline data gathering to turn process into knowledge