No worries about it being your video card or mine, 420! Yours is an Nvidia and mine is an ATI. Different as night and day (mostly). Which means it has to be the door transparency itself and how it works with doors (or not). Transparent textures showing up white typically means the transparency aspect is not working. Usually, this is due to how the texture was made and if the proper procedures for making a texture transparent were followed.
I worked with transparent textures a lot when I tried modding with the MUGEN fighting game engine years ago... they were some of the hardest things to get right, but not impossible. The typical color for them was always a bright one such as white or bright neon purple. As I understand things, transparency has something to do with alpha channels just as Morikahn said... but I've never quite understood or figured out the relation between the two.
Definitely, this is a job for the CEP team to figure out.