Just ran into this situation after a client uninstalled & reinstalled Office 2007 Pro. When Outlook was opened, it complained twice about being unable to open InfoPath because it wasn’t installed.
I found multiple other complaints about this, but no solutions (though some suggested removing and reinstalling Office). A bit of digging with SysInternals’ Process Monitor, turned up mention of not finding the somewhat promising value “DisableInfopathForms,” so I took a stab and created the value as a DWORD under the location ProcMon was reporting. Setting the value to 1 cleared the problem on Outlook startup.
Researching this at home later, I found very little mention of this value. The only useful mention is in Designing Forms for Microsoft Office InfoPath and Forms Services 2007 from O’Reilly, which also points out that the setting is available under Tools, Options, Other tab, Advanced Options button as a checkbox to disable InfoPath forms (as well as at HKCU\Software\Microsoft\Office\12.0\Outlook\Options\Mail where I created it).
According to the book this option is supposed to default to on (presumably treating a nonexistent registry value as “1” or true). I’m not sure what happened in this case since the registry value was absent and I’m not sure what the checkbox in the options display was showing.
I think it was probably easier to find this by using ProcMon to look for Outlook failures than by searching the options.[contact-form-7 404 "Not Found"]