I'm not sure what you have tried to do to resolve this.
Is this happening in all the chat rooms or just one
Well, this is what I'd do
First, I'd check my prefernces
In Yahoo
Under "Chat" I'd make sure that "Ignore Chat Invitations" was not checked
Under "Privacy" I'd make sure that "Igonre anyone whi isn ot on my Friend List" was not checked
In MSN
Under "Tools" --> "Options
I would make sure that "Only people on my Allow List can see my status and send me messages" was unchecked
I would check to see that in the chat room, under "Chat Room Options" that "Turn Off All New Whispers and Turn off All Invites" are not checked. Also, in MSN, you need to be a paying memeber to chat, or use MSN as your ISP
If that didn't work, I would uninstall Yahoo messenger/ MSN Messenger, and also go to the folder
C:\Windows\Downloaded Program File
and delete
For Yahoo: "YInstStarter Class
For MSN: " MSN Chat Control 4.5
After rebooting, I would then download and reinstall the newest version of Yahoo Messenger/MSN Messenger
Without knowing the specifics of what you have done, or checked, this is the best that I can suggest. I hope it is helpful