Bot issue when sending larger messages to users on Vista vs XP

I am having a wierd issue with my AIM bot where it sends out a message, but only a user on windows XP actually gets it. If a user is on Vista, the message is showing as sent to that user, but it does not come up on the users screen.

I have determined it has to do with message length - anything more than about 10 lines down will not display on AIM running on a Vista machine, but it will display fine on an XP machine. Is there a workaround for this - the actual message does not exceed the max size at all, in fact trying to send this
A
B
C
D
E
F
G
H
I
J
etc will display fine on XP but on Vista, nothing. If you start making it shorter, it will post eventually to the Vista user when you get below a certain line number. This is just an example to make a point. Does anyone have a workaround for this issue or is it yet another Vista suprise.

Thanks

Chris