motherboard

My PC won't power on. read: not a boot issue, but a power issue. It doesn't even start to boot, no POST, fans won't even come on. The South Bridge LED blinks (instead of steady "on" like normal). It is an ASUS M2N4-SLI mobo.

I initially thought it was a bad PSU. I replaced that, with a 500W Rosewill Stallion. Brand new PSU, same issue. Would bad CMOS battery cause this? I haven't tried the CMOS jumper switch yet, just wanting some feedback before I get really deep into troubleshooting this thing, and miss something obvious.

My next step is to reset the CMOS jumper, which I'm thinking should reset BIOS settings, as well. I will then begin disconnecting drives, cards, and see if I can at least get it to POST and get to BIOS.

I hope it's not a bad mobo. It's still under warranty, but RMA-ing it would be a pain in the ass. If anyone has any ideas to try, please let me know.

P.S. I'm at work, the PC is at home, and I haven't had much time to do anything more than put in the new PSU, which I did last night. Everything is connected correctly, the PSU is switched to 115volts, the ASUS user manual didn't have any diagnostic list for blinking LED light on SB, so I have no idea what that indicates. Maybe grounded improperly? I'm trying to get as much advice/ideas as possible so I can gear up and trouble-shoot it tonight. BamaWISP is coming out to install this afternoon, and I would like to have my rig up and running by this evening, but am not holding my breath.

My setup, just for reference: Asus M2N4-SLI, Rosewill 500W PSU, AMD x2 2.2GHz CPU, G.Skill 2GB RAM (2 x 1GB), 250GB Seagate HDD, XFX GeForce 8600GT GPU, 2 LiteOn DVD drives, Sony floppy drive.