The updates from Microsoft will [should] not have any slowing down effect on your PC. There have been the odd couple in the past that caused a problem but these were soon updated and it was easy to remove the problem ones from your control panel, each one is shown up separate in there as a "Hotfix" with a KB number.
You can connect direct to the update site from clicking on the bubble or from Start/All programs and look to the top of the list were you will see the "Windows update" link. Once you are on the site Microsoft will scan your system and inform you of any Security updates and other fixes that your PC needs as well.
I would say go ahead and download/install the security ones, decide for yourself if you need the others. You are given the choice to tick which ones you want or leave as recommended and let the auto download do the work for you and install them as well. I Personally install all the updates, no problems so far. Hope this helps. j.