We launched a new site a few months back focused on security - and tbh I was pretty underwhelmed by it - a lot of links, very little helpful navigation and virtually no content. BUT.... I am very pleased to say that the site has now been refreshed and is looking a lot more useful for developers looking to Microsoft for guidance on security. Definitely worth checking out http://www.microsoft.com/uk/msdn/security/default.mspx.
I would also recommend you take a look at one specific download - the new Developer Highway Code which can be found here: http://www.microsoft.com/uk/msdn/security/dev_highway.mspx This handbook captures and summarises the key security engineering activities that should be an integral part of your software development processes. Nice work from my colleague Paul. (that would be Paul Maher - as my close colleagues also include Paul Cross, Paul Foster, Paul West and Paul Murphy - darned confusing at times!)