I've just developed a simple NSIS plugin to generate secure random passwords using the Microsoft CryptoAPI. I did this because I wasn't satisfied with the few predictable PRNG options available to NSIS currently.
This plugin consumes entropy from the OS. You specify the length of the password, and then that amount of entropy is squashed into the default 62-character set (upper and lower-case alphanumerics) and returned on the stack.
MessageBox MB_OK "New password: $0"
NSIS plugin wiki page: http://nsis.sourceforge.net/Pwgen_plug-in
The ZIP contains the binary, source, documentation and an example. I'm releasing this as Public Domain. Do with it as you please; no attribution required.