I used a KVM to send text only to my main and then /dnd on my other characters. When I wanted to chat, I would switch (or use a dedicated USB keyboard) and then when I needed to resume, I would switch back.

The dedicated KB is probably best as it saves you from having to (or forgetting to) switch out of talk mode and back into combat mode to be able to do anything.

Just plug a USB KB in and type away. You can have multiple keyboards under XP no problem. Vista too, I would assume.