Brian, what do you have for a computer?
Also, befor you just go and order one, lets talk. I may be able to get you a serial port, OR Radio Shak had those... You could buy one, try it, and if it does not work, return it this weekend. When you stop, remind me to show you the case of cables and connectors I've bought online that are good for notin'
Also, I think prugs is referring to something you have not considered yet... the grounding of the cable from the AC. That's the way I read it.
If you connect it without a proper grounding, which would result if you were able to connect with the serial cable, the computer would be able to properly ground the whatever stray signal voltage that was in the line. If you convert it from that nice, but old serial line to a new, but amazingly simple USB line, you may loose that grounding ability. I would also consider a grounding probe inside your tank, perhaps 2, one in the tank, and one in the sump chamber that has the electronics and the probe. Mater of fact. Ground everything.
Here's why I say that... A few weeks back I had a heater go to heck (you know, heck... the place down south) The seal leaked, and water was able to get inside the housing. It shorted out. but it didn't just stop there. No sireee it continued to be live in the tank. Because I moved a heat run in my basement, I disconnected the power box and unfortunatly the ground probe that was screwed into it. Thinking quickly as all reefers would, I ran an extension cord and plugged in the tanks power bar into an industrial power cord. HOWEVER, the ground probe was still hooked to the dead line, with no grounding. That familiar smell of burnt electricity and electronics soon followed.
I would ground everything so that you don't accidentally turn your new shiny AC into a burnt out toaster.
J