Asus P4S533-E USB issues

I recently built a new computer, using a 1.8A P4 cpu, 512 MB Kingston DDR PC2100 RAM, an MSI GeForce ti4200 64 MB DDR RAM, an 80 GB fluid Seagate IV Barracuda 7200 rpm and an Asus P4S533-E motherboard, using the Sis645DX chipset.

So far, it's mostly been a breeze getting the thing to work. The cpu oc'ed to 2.6 ghz easily and stayed stable and relatively cool with the standard cooler. Save for some Windows XP installation problems, everything went fine...

Except ...

Ever since bootup one, I have had problems with the two USB ports on the board (I didn't install the four additional USB ports that came with the board). My USB DSL modem refuses to start unless I reboot my computer, after which it works fine (the frustrating part is that a reboot is required after every bootup), and my USB flatbed scanner, though it's turned on, is not recognized at all.
Installing the USB 2.0 driver as well as the USB patch from Sis' homepage didn't help. I'm positive it has nothing to do with my overclocking, since it has the same issues at standard clock. My old computer had no problems with mentioned accessories, so I'm having trouble seeing what the problem is.

I have looked through several reviews and QA's, but I have not been able to find an answer to this strange problem. If anyone here could help me, I'd be extremely grateful.

(Note: I am aware that another topic about the P4S533 exists on the board, but since I thought this issue was a bit different, I went ahead and made a new topic for it. If this upset anyone, I apologize)