You Should really install those updates, if you don't know how to secure your PC properly otherwise..
Crashes/reboots: This is not normal behaviour..
but: LOTS of people experienced this after installing the security updates from April 2004: see google or PC-sites..
do you maybe have a cd/DVD-Writer ?
problems have been reported about crashes after XP-Update Ms04-011, especially if there are some CD-Writer-Drivers for direct access or similar involved.. can you disable those ?
you might want to try downloading the Win-updates manually, and installing them in safe Mode
also beofre do a diskcleanup, scandisk & defrag, and reduce the amount of starting programs/unnecessary services.. maybe this helps..
of look in the MS-Info about the updates, if there is a workaround