You can change the voltage by going into the BIOS and Softmenu III, you'll see the options for voltage, just select user define and set the number. I'm not positive but i'm fairly sure you need to have unlocked your chip to change this.
I haven't heard that changing the voltage can help with VXD errors. I know that if you're overclocking and the voltage is too high windows may throw some VXD errors at you. Either way, you shouldn't really be changing the voltage unless you're overclocking and it doesn't look like you are. So I would say the default voltage is a good one and your problem is elsewhere. However, I could be wrong. Getting the right voltage is just a trial and error thing.