Overwriting the defualt system printer is adding a dependency on to any application you are writing. There are a whole slew of testcases that you would want to verify before rolling out this sort of form.  For instance how do you handle the case when the user does not have permission to create a printer connection and the printer that you specify doesn't exist.  It is certainly not a one liner of just setting ActivePrinter.  I know that Word and Publisher support this sort of Object Model.  It's not hard to do, just not trivial.

Using InfoPath you can setup a print view to use Word to print the form.  Basically, InfoPath uses XSL that is created by Word and the Word Print engine.  So that you get all the great data captureing benifits of InfoPath and the great print fidelity in Word.  You might be able to specify the active printer via Word in the Word Print view.  I'm not sure but it may help.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/ipsdk/html/ipsd
kUsingTheInfoPathToWordWizard_HV01119047.asp