The below error message pops up when i try starting this game. I'm using win 7 64 bit os.

An Assertion failed in F:\Torment\Source\chitin\ChVideo.cpp at line number 4322
Programmer says: No valid video modes supported.

It could be the same problem in this thread:
http://www.gog.com/en/forum/planescape_torment/am_assertion_failed_in_f_torment_source_chitin_chvde_cpp_at_line_number_4322_pro

Beyond that, I don't know, as I don't use Win 7.

Right click on the shortcut and in the compatibility tab :

- select Windows XP (service pack 3) mode
- disable visual themes
- disable desktop composition
- disable display scaling

If that doesn't help, try running the game in windowed mode :

- edit your torment.ini file and find the line that says “Full Screen=1″ - change it to “Full Screen=0″. You may need to set your desktop to 16 bit as well.

If you install mods, save a game, uninstall mods, load up the previous save and try and enter certain areas, you also get an error message starting "F:\Torment\Source" - regardless of where Torment is installed. If that's what you've done, you need to make a clean re-installation of Torment and re-install the exact same mods you have before, then load up your old save.

I solved this problem by installing the widescreen mod using 1280x960 (4:3) resolution. For me the game runs filling the whole wide screen anyway. Movies still play in a small portion of the screen but that's hardly a problem.

P.S. In my experience it's best to install all mods you want to use first, and start a new game after that. For example,I wasn't able to use my old saves (game crashed upon loading) after installing the tweak mod

I've restarted a game playing it in the default resolution but with the other mods installed. Partly because I suspected it was the UI and/or resolution mods that were making it crash more or less every 1 in 5 loading screens, but mainly because it seemed weird seeing TNO and co. being all tiny, like units in an RTS. I much prefer their big, chunky forms - how I always think of them when I remember playing PST all those years ago.

Anyway, since doing that it hasn't crashed once from loading a new area (I've got to the point where I'm off to the location my body was found) but it's crashed twice from using the quick load function - something it never crashed from before. How strange...