Hey, thanks for replying!
I don't think the jumper cap is worn out - I've only had this mobo for a few months. OTOH, anything is worth a shot, at this point. I do have another board that I can pirate a jumper cap from, so I'll try that next. I have a backup PSU I'm also going to try. It's not very good - it's one of those ones that came with a barebones kit (can't remember the brand; either ULTRA or Rosewill, I think), but it should be enough to determine whether the problem lies with my current PSU.
I have the next two days off work, so I'm going to start from scratch and see if I can figure this out, so I'm really grateful for help.
P.S. I hope I'm not the only one who starts to go a little crazy without my computer.

I'm using an old 1 GHz laptop with 376 MB RAM - it's a little bit like watching the web pages being painted onto the screen. If I think of it as a digital ViewMaster I can pretend that using it is more fun than the reality...
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2.4 GHz AMD Athlon 64 3700+ Clawhammer - 2.0 GB DDR RAM - Thermaltake Toughpower 650W PSU
NVidia GeForce 7600GS 512 MB AGP 8x - Sound Blaster Audigy 2 ZS Platinum Pro
1x250 GB SATA - 1x120 GB SATA - 1x120 GB IDE