I may update this post with pictures or just write it a bit more coherently, but this will have to do for now. There are mainly two versions available on the web, the demo and the CD-rip. They both have pretty much the same directory structure and files, with the obvious exception of the demo being smaller and having a lot less levels.
Also, the CD-rip can be found as an ISO file, and this tutorial is based on using that version.
Disclaimer: As far as I've tried, I haven't been able to save a game while playing in Windows XP, for reasons mentioned in the following text. My best solution for now is VirtualPC with Windows 98 or 95 as a Guest OS and using WinThrottle in your Windows XP to slow down the CPU.
The game itself asks for the game CD in drive to be able to start. There is a Start.bat in the package that tries (and fails) to create a vritual drive A: (subst command) but the important thing is that it also copies the havoc.ini into C:\Windows.
As much as I understand, this copied havoc.ini in C:\Windows is the one that the game actually reads. But even if you change the path in that file to the game files on the HDD, the game still asks for a CD. So the trick is to put the path of the virtual drive where the ISO is mounted.
So the simple thing is this. You mount the ISO, start START.BAT, go to C:\Windows and edit the HAVOC.INI and put the correct path of the virtual drive. Then copy the whole contents of the CD to the HDD, preferably to the root of the HDD, something like C:\Havoc. Then start the game through the EXE file in the C:\HAVOC\EXE\ folder and the game should run.
However, there is another thing. I use this method to start the game in Windows XP. I have also installed Windows 98 in VirtualPC, and the situation there is a bit different. The method is the same, but here, the game creates a HAVOC.INI in C:\Windows and there is NO PATH INFORMATION in that INI file. However, the files you copy on the HDD have the havoc.ini and it seems this is the INI file the game detects. So you have to put the HDD path of the game in this INI file (but you still need the CD mounted anyway). This way the game detects a CD (ISO) in drive and then reads the HAVOC.INI from the game folder.
I find this confusing, don't get it myself (how it reads differently on different OS). But it works. Needless to say, there is no installer in this version (or any version I've encountered).
The main problem with the game is that it's too fast on modern computers. Also, playing it on Windows XP, I've encountered a problem with saving the game because the game reads the havoc.ini from C:\Windows which points to the CD, so the game tries to write the save game on CD and fails.
So, I ultimately played the game with a combination of VirtualPC and WinThrottle (a CPU slowdown program). I am perfectly able to save the game (the saves are in the HAVOC\EXE\ folder) and have managed to complete it.
I apologize for the long and somewhat confusing post but I wanted to make it as informative as possible so other people could play this game.