Are there any commands or tools for sending messages from Ubuntu to Windows XP that don't require me to install any client software on the Windows PC?
I'm looking for something like
net send in Windows XP.
You can use Samba in Ubuntu to accomplish this. Simply type:
smbclient -M computername
where computername is the netbios name of the pc you're sending the message to. You can then type your message and press Ctrl - D to send.