SP3 generally fixes a lot of problems - security related issues especially. On expert forums such as BleepingComputer's it is recommended time and time again that users install this upgrade. It is strongly advised that you do NOT run a computer on outdated Service Packs such as SP2 for XP Home.
And yes: Windows Installer has been upgraded as well. You should install SP3 immiately after having run at least 3 different security scannings. Full scannings, that is. Always make sure that you install new programs on a clean PC.
Malwarebytes' Anti-Malware: http://www.malwarebytes.org/mbam.php
Recommended online scanners:
BitDefender Online Scanner: http://www.bitdefender.com/scan8/ie.html
F-Secure Online Virus Scanner: http://support.f-secure.com/enu/home/ols.shtml
Windows Live OneCare safety scanner: http://onecare.live.com/site/en-us/default.htm
Just running your ordinary security programs is not enough. You'll need extra precautions for a download as important as SP3.
Edited by ePost, 10 January 2009 - 01:47 PM.