Why am I not able to sign in to yahoo messenger?

Please don't ask me to check my network connection. I have done all that. I am able to log on to webmessenger so it is not an issue with my account also. The problem is clearly with the client. I have uninstalled and reinstalled several times but no avail. I was able to use it till recently. I tried setting DEP but that didn't work either. I am totally clueless as to what would have gone wrong. Please give me some useful info. I tried installing Yahoo messenger 9 and that gives an error no. of 8100604 or something like that but nothing more. No help absolutely for messenger. Could someone please help?