Windows priority updates cause system slowdown

Hi,

I have a very strange problem with one of my computers. It is a AMD XP2600+
machine with 1 GB of RAM, running Windows XP Professional with SP2.

Whenever Microsoft releases priority update for Windows, the system becomes
slow. Starting a program that normally starts in no time suddenly takes >10
seconds (without any processor or drive activity). And this doesn't happen
when the systmen is downloaden/installing anything, it just happens when
Windows notifies me that updates are available. When I set it to
auto-download the updates, the downloading process takes days. I do not use
auto-install because I do not like the forced reboots, so I don't know if
this too takes very long (this system functions as file-, print- and
webserver and workstation). When I disable automatic updates entirely
(including the warning that my system is at risk), the problem still exist.
The only way to make the problem go away is to manually download the update
and reboot. The problem then goes away, until the next time Microsoft
releases updates.

Any ideas on how to solve this without reinstalling Windows?

greetings,
Erik