Here's the link to SP6a. (This is for Standard Version for Intel (x86) platform - U.S. English):Windows NT 4.0 Service Pack 6a
I would proceed with the SP6a install before attempting anything further.
What is the make and model of your computer? Did it come with a CD/disk with a driver for you video? You can always use a program like SIW 1.59
to find your system information.
Is there a reason that you installed NT 4.0? Don't get me wrong, it works quite well once you get it installed and all the drivers loaded. But, it was mainly used on business computers. We used it at work for a few years. After that, we switched over to Windows 2k, and will be switching to XP soon. On a home computer with a Pentium II based computer, my preference would be Windows 98. Although, Microsoft is ending their support of 98. See here.
Anyways, let me know how you make out with the SP update.
Hope that helps,