Some who have installed SP3 are reporting trouble with third-party firewalls (especially Zone Alarm) and with several anti-virus programs. Not all of their troubles were easily cured by reinstalling the programs. Users who disable these programs while installing SP3 seem to have fewer problems. A few tips for your friends might be:
1. Temporarily disable your firewall and enable the Windows Firewall
2. From the task manager, disable all running applications
3. Don't browse the web or open other programs while downloading and installing SP3
4. Reboot immediately after installation is done
5. Disable the Windows Firewall, enable your own, and check all your security programs on reboot
The extra-cautious may prefer to install from CD; then you can unplug your network cable before killing your defenses. An ISO (545 MB!) for the install CD is available at http://www.microsoft.com/downloads/deta ... laylang=en
I've installed SP3 without incident on both my desktop and my laptop. I use Comodo and Avast on one, Zone Alarm and AVG on the other.