What do you mean by "Explorer form"? Outlook custom forms are applied to individual items, not to folder UI. The methods available to change the appearance of the Explorer window include folder home pages and, in Outlook 2007, custom task panes.
As Dmitry said, you can pretty much put whatever you want into an FHP. See http://www.outlookcode.com/news.aspx?id=45 for links to samples using .NET
techniques.
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Sue Mosher, Outlook MVP
Author of Microsoft Outlook Programming: Jumpstart
for Administrators, Power Users, and Developers