[ajb]

It says in the OP. Windows at the time did not provide a cryptographic quality random number source.

[denton-scratch]

I saw that, and wondered why PuTTY didn't contain it's own good CSPRNG, something like Fortuna, if Windows didn't offer one.

[ajb]

You still need a source of entropy, which is easier for an OS. An app has to resort to the user moving the mouse or bashing keys, which is a worse UX, although I guess they did that for actual key generation (if PuTTY did that) but it would be annoying to do it every time you made a connection.