No, the code behind an Outlook form can only be VBScript, and the form must be published to run any code at all.
What you want to do is certainly possible. The event you'll use depends on whether the combo box is bound to a built-in property, a custom property or no property. See http://www.outlookcode.com/d/propsyntax and http://www.outlookcode.com/d/formcontrols.htm#listcombo
FYI, there is a newsgroup specifically for Outlook forms issues "down the hall" at microsoft.public.outlook.program_forms or, via web interface, at http://www.microsoft.com/office/community/en-us/default.mspx?dg=microsoft.public
.outlook.program_forms

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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
>I have been programming Access and Excel for years and now doing some
> Outlook - and hating VBScript - too BIG of a learning curve.
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> box, then unenable the list box. There does not seem to be any way to
> do either. I have tried several examples and nothing works.