Is it possible to run Windows 7 from a read-only hard disk? I know that you can move the users folder to another drive, so you could in theory run Windows 7 in Read-Only mode, which would make it much more reliable and secure. Linux can do that, can Windows do that too?
Windows needs constant R/W access to local disk C:. (using SYSTEM account, and various SERVICE ACCOUNTS) Your user should not require R/W to anything but PROFILE. Unfortunately NTFS does not allow extended attributes and access controls. I am sure there is an equivalent way of accomplishing this in Windows, but I cannot imagine it making a difference from a security stand point as most nefarious writes are from escalated privileges or spoofed accounts.
EDIT: Look into something like Deep Freeze which will restore back to a specified state no matter what the damage. I am not sure of a free alternative, but the above link lists some.
You can create a Windows PE setup which typically runs from a bootable CD/DVD. It of course uses a RAM disk to write things during run-time but none of that persists between boots.