[tt]^[]0;somestring^G[/tt] is the standard terminal control code to change an xterm title, and since PuTTY uses xterm style emulation it works for PuTTY too.
Normally you would use something like [tt]`tput cap`somestring^G[/tt] or similar for portability where "cap" is the capability name. This would make it read the capability from the terminfo database and use the correct control code for the current terminal type, or none at all if it doesn't support that feature.
Unfortunately I've never been able to find the terminfo capability name for changing the window title, so I'm stuck with the manual method.
Annihilannic.