There are two versions of the Adaptec 2940/7800 family drivers in SP4. The update chose the wrong one.
The AIC78XX driver is located in the \Winnt\System32\Drivers folder. You need to replace it with the "other one" from Service Pack 4. The two files are:
AIC78XX.sys 26,704 bytes
AIC78XX.001 56,272 bytes
Boot to an alternate install of Windows NT.
Inspect the size of the installed driver in the original installation.
Replace it with the "other one", renaming it to AIC78XX.sys if necessary.
If you have a c:\NTBOOTDD.SYS, copy the new driver to that name also.
You might want to build a Boot Floppy.