You've probably already read some of the interesting issues brought about by installing SP3 on your up-to-date XP Pro SP2 system. In an effort to avoid some of those issues I downloaded the entire ISO and installed that on my last personal Windows XP Pro computer. Well lucky for me I was smart enough to create a restore point and make an image of the drive before running the install of SP3. Saved me an incredible amount of work.
Turns out that if you've installed IE7 on the system, without un-installing it, it regresses the system back to IE6 with some sort of weird between state with the mix of IE6 and IE7 files left. The worst problem is that the un-install option for IE7 is greyed or locked out. This behavior was reported or verified by third parties other than Microsoft. I managed to get a bit of a heads-up in that one test system at work exhibited this same behavior with a Beta release of SP3 that did the same thing. IE7 was originally installed as a suggested update through Microsoft Updates.
After making a ghost of the system and removing IE7, I installed SP3 on my home WinXP Pro system that I religiously kept updated. Everything except Remote Terminal Services and Windows Messenger are updated regularly. I removed both of them the day after the very first time I installed XP 7 or 8 years ago. I have never allowed RTS to operate, ever. Likewise Windows Messenger has been removed and shutoff permanently. Both of the services have been repeatedly removed each time the updates have re-installed them. I consider them totally irresponsible components that Microsoft should have never allowed or released in the operating system. DirectX, ActiveX and ASP.Net should likewise be un-allowed as standard components of the operating system. As add-ons for specific purposes, fine, but they should be installed separately and as part of other products.
Turns out that I'm still finding surprises that may or may not be related to the SP3 install. It most likely has to do with the fact that I have VS2005 Standard on the system, VS2008 Express installed, Eclipse, various Web developer kits (some Microsoft, some not), FireFox 2 and Firefox Beta3-5, various C# and assemblers for Windows, and cross-compilers for C and various assemblers for 8 and 16 bit embedded processors along with Virtual PC 2007. I can practically guarantee that Microsoft has NOT tested this combo of programs and IDEs, so as much as I would like to, I'd like to blame them but I can't. I was ready for it this time though and I got my old XP box back after putting the ghost image back.
A lot of strange things happened. For instance, VB.Net and ASP.Net project files written in VS2005 came back as associated with VS2008 Express.
IE7 now just flat-out disappears when I attempt to download more than 3 HTTP files simultaneously. No messages in eventviewer. No annoying message boxes. Just poof, its gone. When I restart it, it acts like its starting up from scratch even when the settings are set to restart the prior session. It acts like its short-term memory is gone.
FrontPage 2000 no longer works properly. Web publishing even to web sites that use FP2000 extensions no longer works. That's likely due to the Web application Express 2005/2008 installs.
If Microsoft wants to be responsible and attentive to its shrink-wrap and OEM retail customers, it will do a “make-over” of SP3 and fix the IE7 uninstall/reinstall issue and do it soon. I still think Internet Exploder is the worst browser of all of the current browsers, mostly because of bloat, but it beats IE6 which was a disaster.
For the non-technically gifted or trained Microsoft users, SP3 is scary and likely to break something the typical user can't fix.
So until I can figure out what I have to un-install and re-install after installing SP3 on my old image and making everything work reliably, the SP3 image is on one hard drive and the old SP2+ updated image is on another hard drive. I've edited the boot.ini file to allow the system to dual-boot the “same-OS” in 2 different locations with two different service packs. Should be interesting.
My recommendation? Don't install SP3 on anything you need to make money with until you test it thoroughly on a system configured like the final target system.