P5N-T Deluxe - USB problem and MCP voltages

My mobo plays dumb when I try to use USB. Every device that I plug while mobo is shut down, works: mouse, keyb, card reader, gamepad, pendrives, etc. But, when I boot into OS, mobo stops noticing that any device has been plugged into usb. Any working-so-far device, unplugged and then reconnected (doesn't matter if same port or not) isn't recognised - mice won't light up, led on pendrive won't blink, but power is on - I can still charge my phone and external hdd will spin up. It's all the same, doesn't matter if it is Vista 32/64, XP 32/64, Server 2k3 64, Fedora, Ubuntu or Slackware - I suppose then it's a hardware problem.

System specs:
C2D E8400,
4x1GB OCZ platinium2 800-4-4-4-15/2 2.1V,
2x 8800GTS/512 Asus
4 HDDs, 2 DVDRW burners, all sata
BeQuiet DarkPower Pro 1200W
rig in CM Cosmos 1100, properly air cooled.
I started with BIOS 0303, ended with newest 1402 (I believe.) and all nForce drivers that are available on nVidia and Asus website.

Any ideas how to deal with this?

Another problem is with north/southbridge stability. The mobo lacks any OC'ing potential (going fsb higher then 360MHz result in certain crash during OS loading, sometimes few seconds after booting to desktop, going 380 results in no post, with lowered cpu multiplier, of course!). The problem is I don't know what are 'stock' voltages - of SPP, MCP, and the dreaded BR04 (aka nForce 200). There is no info on this, and I'm not satisfied with answer 'stock voltage is 'auto'.', which I got from Asus tech guy. Please help me!