i'm not sure how much if any of this will apply since it mostly concerns wavelab, but here goes anyway.
1st tho, iirc, the ua-5 manual says something about always using the same usb port as you used when installing the ua-5. i always either got it right or it didn't matter? also, i always make sure to have wl turned on and ready to go before turning on the ua-5.
i also ran into an issue were i couldn't pass a signal and spent the whole morning of a festival trying to get it right. turns out i had to switch the sound card from one thing to another. can't remember what to what and my lappy's broke so i can't give it a check to see. i did have to make this change every time afterwards in order to use it.
this was with media player(?) instead of xp but shouldn't be any different.
i know, not much help, but it's a couple things to look at anyway.