virtualbox.org

I figured out my processor has VT-D capabilities, so I turned it on in the BIOS, then started my XP virtual machine in VirtualBox 3.0. The first time I tried I could barely type in my password to login before it froze and I hard powered it down. Next time I did it i typed the password in, hit enter, then it froze.

I thought VT-D was supposed to improve performance, not cause issues?

The problem can be that the guests are 32 bit single processor and when you turned on the VTx/AMD-v you are trying to run the guest with the wrong kernel now.
Windows may see this and try to install the right kernel, but you need to make sure that everything is set right before hand.
There are plenty of help pages on the net on how to change the Windows kernel. Google install smp kernel in windows xp

vt-d <> vt-x, only vt-x can be used.

I'll have to check in my BIOS again to see which it is vt-d or vt-x.