XP SP2 can be up to about 400 megabytes, and it's effectively a brand new installation of Windows XP.

The most notable feature of SP2 is that Microsoft has closed a lot of security holes. Part of them were bugs that were fixed, but a large part consists of reconfiguring the default settings throughout the system for maximum security. That is, the default setting is the one that's most restrictive, and therein lies the rub.

XP SP2 will cause a lot of programs to stop working if they make assumptions about how security-related features are set, or if they depend on some operating system service that by default was always on, but is now turned off.

There have also been random reports of the update not working and leaving computers inoperative. Happened to my new boss, which prevented him from filing some paperwork related to my employment, which meant that I didn't get a computer or security badge last week. As I conseqence of SP2 I had to be escorted to the restroom all last week. (Just thought I'd put a human face on the problem...)

People need to approach this update with caution. It's almost certain you'll have problems of some kind; only the severity can't be predicted in advance. It's vital to prepare for the update by making a full backup of everything, and to be sure that you still have the original program disks to reinstall those that stop working. That won't help with configuration problems I mentioned (though it might solve other problems), so you might also want to check with the Web sites of the vendors of software you rely on, to see if they've posted information about problems with the SP2 update.

Those are the negatives. On the plus side, it's been reported that Microsoft has devoted a huge amount of effort to closing security holes, and SP2 is their vehicle for delivering what should be a huge improvement in security. It's not enough to patch holes individually when they're found; they also needed to look at the big picture of the system as a whole. Fixing stuff in one huge gulp makes it more likely that the fixes will work together and not trip over each other.

You need the update, but you also need to be ready to deal with the consequences.