I did indeed end up verifying that the second drive (middle cable adapter) was set to Slave. The problem I did not anticipate is that Dell evidently doesn't default their OEM drives to Master... they default it to Cable Select. So this may be part of the problem. I think my plan of attack will flow thusly:
1) Boot as-is, see if I can see a nameless drive in the My Computer menu (99.8% sure it's not there; I looked several times across two different sessions, and three logins). If it's not there, see if I can use something analgous to Disk Utility (is there a counterpart in XP among the Admin Tools?) and see if I can format it from the wonder-GUI that is XP.
2) If those fail, try the BIOS/CMOS method.
3) If that fails, open the case, move the jumper to Cable Select and see if it shows up as a raw drive in My Computer. If not, repeat steps 1 and 2.
4) If that fails, I'm going to throw my sister's computer out the window and let it plummet 15 feet or so to its untimely death on her driveway.
...into the light of a dark black night.