I loaded and installed it on the laptop, and had serious problems. I had to start again to install SP2.
It seems that most people have no problem with it, but that a few find it screws their machine up completely.
Its main purpose was to plug some very esoteric holes in some obscure bits of the operating system. I relied on my firewall, plus ZoneAlarm, plus Anti-Virus, plus (etc etc) to protect me from those holes. I left the desktop at SP2 status,and it works fine.
My view: don't bother.
I heard a malicious unfounded assertion that it had been deliberately "plugged" to force upgrade to Vista or son-of-Vista at some future date. I don't think even M$ would do that, but it did raise a grin at the time.