The first thing to do is make sure you have safe copies of all of your "important software and documents" that are currently on your computer.
Installing a dual boot system is quite invasive to your current setup and there a several things that can go very wrong, so make sure you have a good backup before you even think about beginning.
For some interesting reading and walkthroughs of setting up dual boot systems, try the following links:
click here administration/management/mltiboot.asp
If you do things properly all should be well and the potential problems you may encounter will be minimal through good preparation.
If you get stuck or have any problems, post again for further information.