SSO, regarding SP3, personally I think it beefs things up a bit especially if you are not a great 'Auto Updater' like me.
I dont allow anything on my PC to auto update, I rather go to the site and do it manualy, like often I goto M$ and dont understand something so I leave it.
Its for these reasons I added SP3 and my performance definently improved.

Paul, I agree, the majority of us here are pretty technical I reckon, and enjoy tweaking here and there, but there is a limit to how far do you go

fweinrebe, regarding antivirus, well I run AVG and will be switch to something else on the 25th when 7.5 comes to an end.
Basically when and if I play FSX, I disable the internet as you'll be surprised just how many apps poll their websites and these polls cause interuptions however slight, like ICQ6 is a disaster for polling etc.
Antivirus should be disabled before playing FSX especially on slower machines.

There is also a lot of stuff you can disable on bootup that causes minor disruptions in any game.
Start - Run - type msconfig and click Ok, then click on Selective Startup, click the Startup tab and go disable all the stuff you dont want to start with windows next time you boot, makes quite a difference
I spent a lot of time tweaking things for FSX, like I discovered that although I have 4 gigs of memory (3.25) on FSX, my Virtual Memory increased from 1.5 gig to 2.0 gig, so I told windows to use my E drive for that.
I suppose you could also use a 4 gig USB Pen Disk for that, anything as long as it does not use your C drive.

LOL, ever since I got FSX about 4 months ago, I have learnt more about tweaking windows Xp since I got it in 2002