someone2 wrote:
desktop is a NEC age ~1996.
I replaced the hdd and installed a licensed copy of Win '98.

Now the problem is the very common issue with windows '98 and the video.
The monitor only recognizes 16 colors at the lowest resolution. I have
performed the usual video troubleshooting steps without success.

If the monitor is also ~1996, then it's way too new to be limited to 16
colors, regardless of resolution. But at 640x480 resolution, standard
VGA adapters are, so see if the video processor chip supports better.
www.hardwarehell.com has much useful information on vintage hardware,
and driver software isn't that hard to find.