I Gotta Crazy Mbox And I Need Some Help!, MBox/PTLE Not operating properly Options

[lushbudget]

Hi
I am just starting my 1st HD recording project. I spent about 3 months learning my Mac, trying to learn PTLE 6, toying with my MBox.My system consists of the following: PowerBook G4 Ti (867Mhz), OS 10.3.3, PTLE 6.4, Edirol PCR-30 kybd contrl, a 250Gb USB 2 HD (via cardbus adpt.) and some MA-20D monitorz.
Now, I got everything all plugged in and running, started a new session to lay down some acoustic guit and some strings using SampleTank and when I begin to play I get this horrific static! i mean it's bad! I can't seem to quiet any of the gain stages enough to cut it out. I know I'm new at this, and maybe I don't have it set up right, but I just used the "Stereo Mix" option when I made the new session, and I haven't changed any of the Bus stuff (Don't know what that does, really) Sometimes it works for a while, then all the sudden BZZZZZZZ!!!
Anybody know what I've done wrong or how to help me?
Maybe I just need to take a ProTools class or something.
Any help would be so very appreciated!