Administrator password not working (but not incorrect) - possible virus?

Hi All,

I have a rather strange situation here that the very helpful guys at Apple Support haven't really been able to sort out. I have a boot camped MBP, with no recovery partition. I logged in in single user mode and created a root user thanks to the apple support guys, set the password for the root user. Then I restarted, logged out my personal (admin) user, which is set to log in automatically, then tried to log in as the new root user. Again I have the same issue! It doesn't behave as though I got the password wrong, the screen goes light grey for a moment as though it's logging me in, and then it's back out at the screen where you choose which user to log in as.

Anyone got any suggestions? The best suggestion so far (also from Apple Support) was to repair permissions in DiskUtility, which did seem to add more permissions to quite a few files, but unfortunately didn't fix the issue.

Any Apple security guru want to take a guess at what might be happening. I'm wondering if it could be a virus, I don't throw that term around lightly, I know it's rare, I've never had one on a mac, and I'm very careful with what I download and install, however, when I did the repair permissions I did notice that it said something about an SUID file related to RemoteDesktop/ARD something or other had been modified and would not be updated. Why that would have been modified I have no idea, but I could imagine a virus trying to give access to my machine might do something like this.

Thanks,

Jesse