Redirect Malware?

Open DNS can be very effecive in preventing redirects. Read on:

You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful: The User Tip seeks to offer guidance on the main security threats and how to avoid them.

https://discussions.apple.com/docs/DOC-2435

More useful information can also be found here:

www.thesafemac.com/mmg

If this redirect happens repeatedly, on other sites, see:

Eliminating browser redirects and advertisements

If it was just on that one site, it was probably an ad on that site or the site itself has been hacked.

As for whether you've got anything to fear from visiting that site, probably not. The only questions in my mind are: 1) do you have Java enabled in your web browser, and 2) do you have Adobe Flash Player installed?

If you have Flash, is it the most recent version? See:

http://helpx.adobe.com/flash-player/kb/find-version-flash-player.html

If you have Java enabled in the web browser in Mac OS X 10.5.8, it's almost certainly not up-to-date, and thus puts you at risk of drive-by downloads. If you do have Java enabled in Firefox, open the Terminal and enter the following command:

java -version

What version is it?

If you have insecure versions of Java and/or Flash enabled in the web browser, then you would be wise to run as scan with some anti-virus software. I would recommend Sophos for that, as it has very good detection rates and is easy to uninstall afterwards (just run the Sophos Remove app) if you don't want to keep it.

Hopefully nothing is wrong, but would it be a good idea to erase the hard drive?

Thomas may have a different opinion, but IMO that would be overkill.

No, I would concur. If Sophos didn't find anything, erasing the hard drive is almost certainly not necessary.

Would it be safe for others to be opening up attachments from my emails? Or should that be avoided?

There's virtually no Mac malware that is capable of embedding itself in files that you create. The only exception is the Word macro virus. If you allow macros to run in any of your Office docs, then you could become infected. If you don't allow macros, or if you don't use Microsoft Office at all, then you're fine.

As to files that others create on Windows machines, there's always a risk to passing those on without scanning them with anti-virus software.

Also, is it safe for me to be entering my admin password if I would ever want to download/install anything from the internet?

When you download an application and run it on your computer, there's really nothing guaranteed to be safe about that. You have to exercise caution. As to entering your admin password, some apps will not install or will not run properly if you don't. Further, that's dubious protection anyway. The infamous Flashback malware requested an admin password, but if you declined providing it, you still got infected anyway, it just happened a little differently. There's a lot of bad stuff that can happen without ever providing an admin password.