I’ve seen quite a lot of discussion lately on how to ‘defend against nmap’ or how to change the properties of your TCP/IP Stack so your Windows OS appears to be something else (As in you can guess the OS from the TTL value passed back in a TCP/IP packet).
One way you can do this is with Security Cloak.
Security Cloak is designed to protect against TCP/IP stack fingerprinting and computer identification/information leakage via timestamp and window options by modifying relevant registry keys. The settings used are based on the results of SYN packet analyization by p0f. While the OS reported by other OS detection scanners were not identical to those of p0f, testing against Nmap, xprobe2, queso and cheops showed that they were unable to identify the correct operating system/version after Security Cloak settings had been applied.
Note that in order to properly emulate some Operating Systems, the MTU must be changed. While most of these require the MTU to be 1500 (the default for most network connections),depending on your network connection, this could degrade/interfere with your connectivity, so be sure to check your current MTU before applying these changes. It is reccomended that you save all the original key values before using this program in the event that your computer responds negatively to the changes.
You can find the authors page here: http://www.craigheffner.com/security/
And a direct download here: Security Cloak