nafan, thanks for the advice!
Actually, I'd kept working on it after I posted the question, and though I'm not sure this was what fixed it, I reinstalled and chose all the same options, but told it to automatically log me into KDE. I don't want auto-login, so I didn't choose it before, but this time, it loaded KDE as documented. I went straight for the control center and disabled automatic login, and now it gives me the X login frontend like it should.
Unfortunately, now that I've reinstalled, I couldn't tell you how it was
set up, but thanks again for the info. I've seen this happen with other distributions, and I was really curious about how I could start troubleshooting from that point, even as far as just stopping the display manager, so your advice was definitely not wasted.
About the bootloaders though, my experience was with a much older copy of LILO. I tried "fdisk /mbr" back then, but it just severely corrupted LILO, rendering it unusable (and scattering visual elements of the text-based window all over the screen.) It sounds hard to believe, but I suppose MS's interpretation of the MBR was a bit different from everyone else's... it was like LILO was only partially overwritten, and after that, nothing could make use of the disk. I see that the rescue option on the Mandrake DVD has an option to restore the Windows bootloader, but this time around, I decided to install Grub on the same partition that contains /boot/, and copy the bootable code to my NTFS partition so I can load Linux via my BOOT.INI. It was a bit of extra effort, but it's working great, and now I don't have to worry about what happens if I want to reclaim the Linux partitions later.