Thanks for the suggestion, bevills1, but didn't work. Tried twice.

First, booted using Win98 Startup disk
Chose 1 (start w. CD-ROM support)
A:\>unlock C: (This appeared to have worked as no error message appeared.)
A:\>F: (CD drive where 98SE update disk is.)
F:\>setup.exe (got message that This program requires Microsoft Windows)

Second, rebooted w/o using Win98 Startup disk thinking that C: was now unlocked from previous attempt.
Killed AVG service.
From D:, ran setup.exe (I transferred all files from 98SE setup disk to partitioned D drive from suggestion on another forum). Got SU991010 error again. Suppose I could've tried running setup from F: (CD-ROM drive) but seemed futile.