I wound up sleeping until 11:30, bunching up my blankets and pillows into one enormous pile.
When I finally got up, I reluctantly turned on the computer and gave the DSL drivers another whack (after having spent about 2 hours the previous night trying and failing to get it working). After feeling the frustration level grow from across the room, Kat had a brilliant idea: uninstall everything related to the DSL service, re-install Windows 98 to make sure all of the necessary .DLLs are there, and try again.
I knew there was a reason why I loved Kat. After about a half-hour of calm, unhurried mouse-clicking, Windows 98 was fully restored. With hope abounding, I inserted the original DSL software disk (after being told by SBC support that the update they posted may cause driver problems) and waited. Miracle of miracles, the installation dialog appeared, which is a damn sight further than I got the previous day. 20 minutes of that, a quick call to SBC support (my original order had expired, but I was able to bypass that), and I was back online.
Just like that, my headache, tension, and sour stomach disappeared.
Even better, by reinstalling Win98, we were able to fix a few things that weren't working correctly. Everything is faster, including the DSL connection, which was the whole reason why I was installing the upgrade in the first place!
The rest of the day and evening was spent doing laundry and housecleaning in anticipation of my family's visit next week. Not exactly how I wanted to spend an unexpected day off, but definitely better than the alternative of stressing out while at work.