If u have the GOG.com Max Pack Edition, the best way is to run it from Start Menu using Software or Hardware (3dfx) default Mode. In GOG Edition, the Dos Box Emulator is integrated so Carmageddon runs in both Modes using the DosBox Emulation. In case it isn't working or You have the Original Version of Carmageddon, try these methods:
3dfx Mode using nGlide
For the normal Carmageddon follow these steps:
1) Download and install nglide, I used this one http://www.zeus-software.com/downloads/nglide
2) In startmenu search the nglide entry and run the configuration and set the resolution to the one you prefer.
3) in the carmageddon installation dir in the folder doxbox delete (or rename) the file glide2x.dll This is important! If you dont delete this one, it wont use ndglide!!! and it will glitch!
4) Modify the shortcut on your desktop. You have to edit the target to dosboxCARMA3dfx.conf from dosboxCARMA.conf (see third attached file)
5) Start the game and enjoy the madness =)
Windows 95 Patch (Unstable!)
It is a Patch, remade from original version, but isn't integrated in the GOG Version because of it's potancial unstability.
Announcment @ Carmageddon.com
"Our release contains the DOS version by default, complete with both software and 3Dfx/Glide rendering modes. As you may know, however, Carmageddon also came with a Windows 95 patch.
Because this version has got problems with some ATI/AMD cards, as well as not-so-stellar multiplayer support, we do not want to make it available as an actual part of the release itself. This version will allow for higher resolutions, most likely better framerate, but may also be unstable on some PCs, or not work at all.
Basically, we only support the actual DOS release available in the installer and, if you want to try the Windows version, you do it at your own peril. Just to clarify, the patch does not replace the DOS version, so it's safe to use - there's just no guarantee that it will work better than the default version, or at all. "
You can download the Patch as a separate download here:
http://items.gog.com/winpatch_carmageddon_maxpack.exe
Link to Original Carmageddon Win95 Patch Forum
http://www.gog.com/forum/carmageddon_series/experimental_windows_95_patch/page1
If You have still problems You can use ddraw.dll from here http://hifi.iki.fi/cnc-ddraw/#download
Copy it in the same folder as carm95.exe, open ddraw.ini and change the resolution to your liking
Play the game using carm95.exe
To enhance the graphics, use the parameter -hires
original source: http://www.carmageddon.com/forum/topic/21405