copy/paste from
http://shrines.rpgclassics.com/pc/planescape/cheats.shtml Full Installation
This isn't really a cheat, just someting for the ones who don't like to change the CD's during Ps:T. This also improves performance greatly for those with slow reading CD-ROMs, as reading the information locally is faster than having to read it from the CD. This requires quite a lot of space though (about 2 GB), only do it if you can spare the gigs.
Start by copying your Planescape Torment CD's into separate folders on your hard drive, use folders like "C:\CD2" or something. Then, using a text editor (notepad works), edit the "torment.ini" file in Ps:T's main directory (don't forget to make a backup of that file, you never know :P).
Replace the lines:
CD2:=E:\CD2\
CD3:=E:\CD3\
CD4:=E:\CD4\
with the paths to where you copied your CD's, should be someting like:
CD2:=C:\CD2\
CD3:=C:\CD3\
CD4:=C:\CD4\
If you're have trouble with hard-drive space, do it to only one or two of the CD's.
The typical infinity engine game "put the cd's on the HDD and tell it they are there", it eliminates the annoying cd swapping.