You've come to the right place...

Check out the following links:

Microsoft Security Developer Centre:
http://msdn.microsoft.com/security/

.NET Security:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnnetsec/html/CL_SecRevi.asp

Patterns and Practices Security Wiki:
http://channel9.msdn.com/wiki/default.aspx/Channel9.PatternsAndPracticesSecurityWiki

Application Blocks and Libraries:
http://msdn.microsoft.com/practices/AppBlocks/default.aspx

Blackbelt Developer Security Series:
http://www.microsoft.com/events/series/digitalblackbelt.mspx

Security Events and Web Casts:
http://www.microsoft.com/events/security/default.mspx

This should be enough to get you going!!

Please drop me a mail if you know of any other good resource; such as events, user groups, white papers, books etc.

Look forward to hearing from you,

Paul