I copy/pasted "ERROR 999998: Unexpected Error" into the search box of support.esri.com and found an Online Help entry describing it:
999998 : Unexpected Error.
This is an operating system error and can indicate different error
conditions. These errors are not specifically documented in the ArcGIS
Desktop Help system. However, in most cases, the error description
provided with the error code can be used to look up additional
information on the operating system.
If the error message is "Unexpected Error," consider contacting
technical support for assistance. If the message attached to the
999998 error is any other message, these messages are standard
operating system errors that can be looked up under Websites such as
Microsoft's Support and Help Website
(http://support.microsoft.com/search/?adv=1) for more information.
Whether you contact your local technical support, or seek assistance from a forum like this, my recommendation is to provide the details that @artwork21 suggests (i.e. what tool you are using, data type, service pack installed, etc) at the outset, so that anyone trying to help has a better chance of understanding and reproducing what you are experiencing.