Was hapening to me, too. Captured the packets of one of the "attacks".
It's just a windows messenger spam ad.

Message: Microsoft Windows has encounted an Internal Error\nYour
windows registry is corrupted.\nMicrosoft recommends a complete system
scan.\n\nMicrosoft recommends\n\nhttp://www.(taken out).com\n\nTo
repair now for a free download\n\n

turn off your messenger service and you probably won't receive it
anymore.
The source IP is probably spoofed. Dunno, though. report it to the ISP,
and they may check into it.