Ok, disregard the parts about setting up Narrow connections

I didn't know you had bought a Wide Plextor.
The files I have for the Adaptec drivers (which are part of a distro CD that is still on their site if you go to the driver's section) will (should) work in XP. They are the same driver files for NT and Win2K. I have the necessary ones for both my 19160 as well as your 29160.
If ASPI from Adaptec's site won't install then you'll have to search for a non-upgrade installer and try that or find FORCEASPI like I mentioned before (it will install regardless). As I mentioned, you're not supposed to need ASPI for ME nor Win2K, but it does work.
Plextor may be right about XP, but I don't know why MS would have broken this. Plus it's not sufficient to say SCSI DAE doesn't work - it would all depend on the adapter being used and drivers.
Go ahead and fill out an online trouble ticket with Adaptec. Mention to them that someone you know (me) has had this very same problem (but with 19160) and Win2K.
Email me and I'll send you the driver file archive (it's much smaller than grabbing the entire package from Adaptec).
Bruno