Is it really necessary to download the security updates for the Security Center? Will the updates slow my computer down? How important are the updates? I recently reinstalled Windows XP on my computer because it was becoming so slow due to so many programs running on it and I'm concerned that downloading security updates all of the time will make it run slower again.
Slowdowns are most often due to lack of computer resources, in particular memory and free hard-drive space.
That is XP SP3 I hope? Otherwise you might as well not bother worrying because it's doomed from the start.
Updates are what keeps the anti-virus protection and other parts of Security Center up to date. Without them you might as well not bother using protection as it would be next to useless after a while anyway. New infections are churning out by the thousands every week.