InfoPath Dev

I know this keeps coming up but I'm still confused.  I created an InfoPath 2007 form that has code behind that accesses the SharePoint Object model. This form is for inside the company only.  When I originally signed it I received a .pfx certificate and installed it on my development machine.  Set the form security to full trust and choose the certificate that I had just installed then published it.  Now a year later the form has stopped working because the certificate has expired.  Why does this matter if the code and form have never changed?  And seriously if the form is for in-house who cares if the form is signed or not?

Finally is this correct type of certificate and is there a more recent blog that goes over this?  The ones I see were written back in 2004 or 05 and InfoPath 2003 and VS 2003.   

Can someone please help answer my questions? 

Thanks

InfoPath has no way of knowing that your form is for in-house, and even if it is in-house, whether it's trustworthy.  Trustworthy in-house equates to Domain Trust level, not Full Trust. InfoPath has security safeguards to prevent people from using it for malicious purposes unbeknownst to the users.

Certificates expiring is simply the way certificates work.

 I'm not an expert on certificates either, but I have found that if I use the "Digital Certificate for VBA Projects" tool included with MS Office, it creates a certificate that doesn't expire until 2017.  Maybe that could help ease some of your troubles?

would that work even if the code behind is C#?

Yes, it should work for any codebehind language.