Since it's a quake 2 engine game, it could use system based opengl or look for opengl32.dll in the game directory - with the game directory taking precedence. With Vista and above, O.S. paradigm has changed dramatically - especially with 64 bit - so its best to copy the system opengl32.dll into the game directory to make sure it uses the proper *dll. On 32 bit machines opengl32.dll is in "Windows\system32" and 64 bit it's in "Windows\SysWOW64" directory.
Copying the system opengl32.dll into the Quake 2 engine based game's folder has always fixed any problems for me with modern hardware / O.S.s. I replayed it about a year ago when I was running XP 64 bit and had to do this.

Admittedly, Heretic II was a totally different game from Heretic/Hexen, and wasn't really a proper sequel at all. The name probably just got tacked on because Activision figured it would sell better. It was still a decent game in its own right, though, but it was more action/adventure a la Tomb Raider than pure shooter.
I remember having fun with it back in the day, anyway. Bladestaff duels were great fun.

still got my old Hexen CD lying around my house.... haven't put that thing in for a loooong time. good game though

Not sure where my Heretic II CD came from (either Ebay or bought it off a friend) but I remember not enjoying it a whole lot, unlike Hexen (which I am ashamed to admit I'd pirated.)

Yeah, Hexen 2, despite the fact a lot of the backstory is incompatible with the original game, was a very good game.
Heretic 2.....bleech. IMHO they decided to imitate the sucess of DIablo using a Third Person viewpoint and failed.

Yes - Those PC Gamer Demos can sure scratch an itch when needed. I lost all my PC Gamer Magazines in a fire a few years back but the Discs were stored in the garage - still have them all.

Tried that. The one in SysWOW64 didn't work. Also tried the one in System32 (what a misnomer!)-no dice, either.

I'm running win 7 x64 and tried it out - it was doing the same for me. It appears this is an NVIDIA bug but I found a fix that should work. I also tested Quake 2 vanilla opengl under win 7 without opengl32.dll in game directory and it found the system one fine. In the past, I normally just used "Quake 2 Evolved" when I played Quake 2 so I hadn't tried a vanilla Q2 engine in awhile.
As a side note, I also think I made a mistake when I mentioned placing the opengl32.dll from "SysWOW64" directory into Q2 engine game folder - I think you use the one from "Windows\System32" under x64 as well. But since the engine seems to find the system one fine under win7 I guess it isn't a real issue.

I appreciate the effort, but that hasn't worked out for me either. Tried both extension limit settings, conformant texture clamp to off, and even both opengl32.dll files in the game directory (one at a time since two files with the same name can't coexist), with varying combinations of the above, but still no success.
(And just for the heck of it, I tried NVIDIA's own OpenGL extension limit setting in their Control Panel before working with nHancer. That didn't help, as expected.)

Well, the only thing left to try then is older Nvidia drivers?

I just pulled out the 97 disc for Hexen II and the 99 disc for Heretic II - both load and play fine. I suggest an ATI Card ;-)

Hmm, which nHancer trick did you try? There are a couple values here that might work for Quake 2/3 engine games: http://www.sevenforums.com/gaming/46278-fixes-rtcw-single-player-quake-ii-opengl.html

Works fine on my notebook/tablet running Win7 64-bit as well, with an ATI Mobility Radeon X2300/HD 2300 GPU. My only issue there, really, is that the integrated sound codec has no chance of giving me EAX, A3D, or good HRTF surround sound like my desktop can.
However, I've been through so much irritation trying to get notebook resolution scaling to work the way I want other than using old drivers that are BSoD-prone and have black flickers in OpenGL rendering. It doesn't help that DX9 parts like that get relegated to a "legacy" driver structure with much less frequent updates and no official Win7/WDDM 1.1 support.
There was also the time where I got to test a friend's HD Radeon 4850 on Catalyst 8.10 (latest drivers at the time). It performed WORSE than my 8800 GT and glitched a bit harder in some of my older games. However, one of those games has since been fixed, and the other got worse on NVIDIA, so it's probably not as bad now.
I still don't want to go ATI when it's time to upgrade from the 8800 GT, though. Notebook issues aside, I've heard reports that ATI's drivers can't easily force refresh rates on FD Trinitron monitors like I use, while it's a cinch with NVIDIA and custom resolution settings. Then there's the apps that rely on CUDA and haven't converted to OpenCL yet...
I tried both of those values. No dice.
I'm using 197.45. Seems like I'll have to change drivers at this rate, but there are some features I'd rather not lose in the process of rolling back to an older version.

Ah well, I'm still on 195.62 and the fix works for me. I'm also running Win XP 32 bit btw...