I finally had time to figure out what was going on with my Slackware tower. It has been down for so long I forget when it actually started having issues. I believe it began having problems since the beginning of 2009.
The culprit? Either the SATA drive that I installed in it awhile back or the SATA controllers on teh board. It's difficult to tell without swapping the SATA drive out for another. I just disabled it (the drive) for now. It works fine without it connected...that tells me its the drive and not the controller.
I began by trying to boot it up by using a Ubuntu LiveCD. It wouldn't boot up and showed many ATA-based errors in the logs. I tried a different Ubuntu LiveCD (v8.10, I believe)...still, same issue.
It was then that I started focusing on the SATA drive. I just unplugged it and tried to reboot...got a reboot and the system has been running for about a week without any issues.
So, I lost a drive. It's not a big deal, as that drive was hosting Windows, I believe. Weird, because that drive is rather young. I believe its a WD (I have seriously bad luck with that brand). I can do without replacing that drive for now, though.