I can't find the source project files anymore, but I still have the msi & setup if you still want those.
PM me your email if you still want it & i'll send it to you.
If you were wanting to compile the project yourself I can give you a place to start...
On your sharepoint server, go to C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS
The file you want is dsigctrl.cab, extract the files to your desktop or whereever.
From here, I google'd tutorials on creating msi files in visual studio....i don't recall the one i specifically used, but there are quite a few out there.
As a side note, i created the package, but someone else deployed it. We used the newest version of sms (sccm 2007) to deploy & it worked fine.
Also, I think this might only be required for XP (don't know about vista)....i could open forms fine on my Win7 machine without having to install it.
ps: if you don't mind, PM me any return emails you got regarding the email address i posted above....it's my first shot @ setting up my own mail server :)
Edit: Finally got around to posting the msi on my blog: http://www.squenchy.com/post/Manually-Install-InfoPath-Digitial-Signature-Control.aspx