I wanted to try recovering my boot partition from a disk image to see if the problem might just be a corrupted operating system, but had made the mistake of saving a lot of files there so the only thing to do was continue offloading data to the external drive, now with an emphasis on clearing the C: boot partition.
It wasn't easy going, and eventually I resorted to copying larger files one-at-a-time to minimize errors. But inevitably the IDE bus would drop down to PIO mode and I'd have to delete the drivers and reboot > detect hardware > reboot, interspersed with generous portions of chkdsk.
To break up the monotony I triaged the other partitions for my most vital files
. When I got to the partition (G:) that had contained the aforementioned 'sluggish' files, things took a turn for the much worse. Now the entire partition was acting sluggish, and accessing any files on it woulddd HHHAAAANNNGG
With a sinking feeling, I continued archiving files to DVD-R and saving backups to the external USB drive.
After deleting a lot of recent data from all partitions my computer still wasn't working any better. The crash/chkdsk behavior intensified and soon new symptoms appeared. In Device Manager the Microsoft drivers no longer registered as being digitally signed. Replacing them with fresh copies from the Windows CD made no difference.
Looking for clues regarding the chkdsk problem, I opened the boot loader and found (edited for clarity) this:
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="My friggin' C: drive!"
Whaaat? C: is definitely on partition 1, and there's nothing bootable on partition 2 (E:). Editing the boot.ini to find C: on partition 1 rendered the OS unbootable, but I was prepared for that and used the XP installation on my 80 GB secondary drive to fix the boot loader. Everything looked alright in that second installation; UDMA 5 was working and the MSFT drivers were still digitally signed.
(*Reboots to C:*)
Windows needs to verify the consistency of the file system on Partition E:
One of your disks need to be checked for consistency. You may cancel the disk check by pressing any key, but it is strongly recommended that you continue.
Verifying Security Descriptors.
Recovering orphaned file fragments.
Rewriting corrupted bit-map entries.
After finally getting back into the C: partition, I concluded that my 320 GB primary drive was failing and decided it was time to offload my data to the 3 TB external drive
For Great Justice with all due haste.