but you say you are running 5 fans in your case? and one gig of ram, and a 200 gig harddrive? ......350 watts is way to low to power all of these items.....i would suggest diconnecting all fans but your CPU fan and try it like that.....if it seems stable, try connecting the CPU fan as well....keep connecting your fans until you reach the level at wich your computer acts up again
this might not be the problem, but it's a good troubleshooting start
have you tried running a stress test before installing the security updates?
your 350 watt power supply may not be enough to supply power to all those componets...
your CPU alone can pull 40 - 50 watts, leaving 300 watts to distibute to your 200 gig hardrive (wich requiers alot of watts) your 4 memory sticks each depend on the power supply to run, as does your graphics card and all 5 of your fans....now does 300 watts seem effiecient enough to power all of these devices?? sure it might start up fine with no problems...but the moment you start putting your devices under more and more stress.....(example headers on HD running to run the windows update installation) can cause the HD to pull additional wattage from the power supply...well if the supply is limited than your HD starts to "borrow" power from other comonnets (example= your CPU) since your CPU requires at least 40 watts to run stable, it may start freezing when your other devices begin to use more and more power.
so i suggest you temp. disconnect all fans except CPU, install the updates and see if it still freezes....
as i stated earlier, the power supply may not be what's causing the freeze ups, but it's a good thing to test first....if your system still freezes after diabling fans, then you can rule out the power supply causing the problem....until you try this, you'll never know
It's still an unknown brand, and 550Watt is far more than you nedd in any case. I'd advise a good Antec or Enermax. A 350W or above (unlike cheapo PSU's, good quality ones actually do put out their rated output) ought to do you.