Error 999 is mostly experienced by the subscribers to Yahoo services. When they try to open any service they are greeted by this error. Here are some ways to fix error 999 in Windows 7, Vista and XP instantly.

Clean You Internet Browser
Open your internet browser and clean all the cookies, cache and history files. It will fix the error 999.
If you are using Firefox follow the steps below:
Remove Registry Errors
In order to fix error 999 in Windows 7, Vista and XP fix Windows registry errors. Click here to fix Registry Errors.
Turn Off Proxy Server
Error 999 may also appear if any proxy server is turned on. Turning off the proxy sever may fix the problem. To do so follow the step below:
Install a Different Browser
Another way of fixing error 999 is to install a different browser. If you are using Interent Explorer you have the option to install Firefox, Opera or Safari. Installing a different browser fix the error effectively.