I had a lot of trouble with off-brand USB 2.0 cards in all (3) of my BP6 systems. Blue screens with IRQ_NOT_LESS_OR EQUAL. Had to pull them all. Since then have had good luck with ADAPTEC USB 2.0, and I think they have better drivers. I'm using WinXP-Pro though, not W98 or W2K.
As I said earlier, I had nothing but trouble with the off-brand cards.
They are stacked in my basement.
Once I got the Adaptec card, all my troubles cleared up.
The slot you use can also make a big difference.
Have you tried shifting?