I changed the permissions on the c:\windows\temp directory to test the recommended NSA file settings against Office SharePoint Server 2007. Guess what happend? Yep, it broke.
When you install OSS 2007, there are 3 security groups created - WSS_ADMIN_WPG, WSS_RESTRICTED_WPG and WSS_WPG. I am still researching what each of these does, I will post that when I find out.
Bill Sheldon talks about them here http://blogs.interknowlogy.com/billsheldon/archive/2006/05/22/2705.aspx
Here are the minimum permissions changes to the temp directory required for Excel Services to work.
WSS_ADMIN_WPG = Full Control
WSS_WPG = Read&Execute, List and Read
I can only assume there will be issues like this throughout the system, so please be careful changing default permissions.
-ben