[yellowleaves]

I have 2 dead orchids and one more about to go. I received 2 orchids a few months ago and recently bought 2 more because I loved them so much. I thought I was doing great but after the flowers dropped the stems became yellow and shriveled so I researched this and eventually decided to cut them off. Does cutting off the flower stem cause leaves to yellow and drop off until the plant seems dead? or maybe they need to be repotted? I have tried reseraching all of this, but I can not really figure out what has caused them to be so yellow and die. There is a lot of info on the web but I have been looking for a good book too. One that is dying right now seems to have a tiny moldy soft spot at the base and the other one has roots growing upward, shriveling up. The leaves looked very healthy at first but then they drop off one by one until it is just a stump. so very depressing considering they were a gift meant to cheer me up; but I do not want to give up. I have one orchid that I cannot exactly identify but I keep it moist in the window and it is thriving. i think the ones I have killed are the phalenopsis variety. I water only once a week, am very careful not to get the leaves wet, but not so much water that they drain. they have good light - behind a curtain in a south facing nyc apartment window. what else can I do? why is this happening? Maybe itis too cold y the window? I have tried keeping the water tray under them too for humidity. Nothing is helping. This is SAD!