Help me UNDERclock?

I'm having a TDR issue with Windows Vista 32 and one of the solutions that I've heard come up most often is underclocking all aspects of the system. I was hoping someone could sorta walk me through it because I went through my BIOS and it looks like most of the settings only have one option selectable. (Vcore for instance).

I'm really new to all these things, and I've read "The Memory Faq" but I need more help, maybe I need to download a software to adjust these settings from the desktop? Anyways, here are my system specs and I'd like to bump the CPU, GPU, and Ram Speed all down a notch.

AMD Athlon 64 X2 Dual Core 3800+ 2.0 ghz
Nvidia GeForce 7800 GT
CORSAIR XMS 2GB (2 x 1GB) 184-Pin DDR SDRAM DDR 400 (PC 3200)

I can't figure out how to tell you which version of BIOS I'm using but the options for changing the voltages and speeds seem to be pretty limited at first glance. Let me know if you guys can help. Thanks.

 

What motherboard have you got?

 

Sorry, left that out. Motherboard is an Asus A8N SLI Premium

 

The Asus A8N SLI Premium is a fine board for tweaking anything. The manual that came with it is fairly complete, but if you've misplaced it you can download a copy from ASUS here. Notice, that the page has a drop-down box for Operating System. If you are going to download any motherboard drivers, selecting your OS first will be helpful. You might also gain some insight into tweaking the settings from this link. I own the board myself, but after reading that article I learned the facts about a couple of things that were less clearly defined in the manual.