If you do have the 3/10/2008 patch, it will be listed in the Add or
Remove Programs window, which you can access from the Control Panel
(in XP, anyway).

You have to select the "Show Updates" checkbox at the top of the
window, then down toward the bottom there is a section for "Windows XP
- Software Updates," under which are listed all of the many, many
patches and updates installed.  At the end of Hotfixes and Security
Updates are listed the knowledge base numbers, so try to find the
patch numbered KB946983 and remove it.

If this doesn't help, you can go back to Microsoft Update and it will
give it to you again.

If you have Automatic Updates configured, you might get the patch back
automatically later, unless you expressly select not to install it.
When you go through the web interface for Microsoft Update, you can
deselect it and tell it not to remind you about that update again.
This patch closes a security vulnerability, so you might want to try
to install it again later or in safe mode or something, but obviously
if it prevents you using the software as intended, it is no good.