It's more likely your ISP. It's complicated, but if you have a 2nd local number that you can use to dial in, rather then the one you regularly use, odds are you'll be able to get into the room once you log in that way.
A quick test to see if the problem is your machine is to go to a yahoo club and see if you can load a chat room there. If so then it's the ISP, and doing the above should fix things.
Myriads