@Scottath: Nope, like I said, the old rig was REALLY old.
@EuphoricMix: I couldn't even get in to BIOS, that's part of the problem. I have all the jumpers right, You're partially right, though - I figured out that it was trying to boot from the external I mentioned before.
Apparently when I last cleaned it, one of the hidden files that designated it as a secondary drive was deleted, so it tried to boot as a primary. I had tried to run the computer without the drive, but it must have still been looking for it. After unplugging it, doing a CMOS clear AFTER that, and starting without it, it worked. I checked the BIOS, and it has an option to boot from USB HDD built in, so I guess it had switched itself over. *Shrugs* Who knows. Working now, though!
Edit: Now I seem to have another slight problem, though - my IDE HDD and Optical Drive aren't showing... wonder what that's all about...