this does not sound like the OSX.RSPlug.A trojan, it does not do any damage to your machine, just changes the DNS settings.
I suspect that you shut the machine down by pulling the power cord or holding the power button down for a while.
You may have noticed that the software update procedure for Leopard is different than the Tiger version. In Leopard, it does exactly as you saw. It downloads the files, restarts and then installs them. In Leopard you get to see the install process happening.
What you have done is shut the machine down in the middle of the install process which leaves the system in a very unstable, and possibly, unusable state.
Try to reboot the machine, it will attempt to rectify any problems, and see what happens.
Worst case scenario will be that you have to do an Archive and Install to put a new system on there. No biggie.
On the subject of Anti Virus. I really hope that you did not mean that you have NO anti-virus installed at all. Please tell us that you have complete AV coverage of your Windows setup.
Let us know how you got on.
I just got a new iMac with Leopard installed and clicked on a link that was very similar to this (that I found quoted at chinwong.com/index.php/site/comments/mac_attack/):
""OSX.RSPlug.A, has been found on a number of pornographic Web sites, the security company Intego reports.
?A great deal of spam has been posted to many Mac forums, in an attempt to lead users to these sites,? Intego warns. When they arrive at these sites, they will see still photos, purportedly from free porn videos. But if they click on them, they will receive this message: ?Quicktime Player is unable to play movie file. Please click here to download new version of codec.?"
***It was not a porno site, but was in fact a google search result that was shown as linking to my website! It downloaded 5 "setup.exe" files, which I deleted, and forgot about. Then I went to create a .Mac account and it said I should do a Software Update, which I did for two updates (and for which I had to enter my administrative password) it ran the updates and said I'd have to restart, to which I said yes, but a Windows Crossover program was running at the time which cancelled the automatic restart, so I was prompted to quit the program and manually restart, which I did . When the computer restarted it said I had two updates that needed to be installed, which I thought was strange, as I thought they had already been installed, but I said okay to that anyway. Then it started progress bars and was taking a long time, saying, "writing files", which it did to 100%. Then it said "patching files" , and I thought that was really fishy, as I'd never seen anthing like that before on a Mac, so I shut the computer down. When I rebooted, I got the gray kernel panic window that says "you need to restart your computer. Hold down the power key for a few seconda or press the Restart button." in four different languages. I unplugged the computer and an external back-up drive.
What should I do now? I don't have any anti-virus software installed.
Any assistance would be more than greatly appreciated! I have tons of data on the drive that is not backed up! Many thanks in advance.