Encountering Error 999 on Yahoo Messenger

Hi,
I am encountering an issue with Yahoo! Mail. The error message is
Sorry, Unable to process request at this time — error 999.
It says that my request couldn't be processed at this time and that I should try again later.
This is my first time to encounter this issue. It says that for me to be able to resolve this, I should look over my system and scan system for a spyware and virus. I did what it said. I first updated my Anti Virus program from the internet and then started scanning the system. It was completed with no virus or spyware found. It is also posted that the error is cause by unusual activity coming from the Internet Service Provider. I can't think of such.
Please help me surpass this error as I really need to create an email account in yahoo for a project.
