I was just like you too. When I first learned linux I screwed up my computer as well. But that's a lesson that you will appreciate trust me. I have reformatted my computer at least 10 times while trying to install linux. Here, do this.
1. Run fdisk and split your 120gig into 2 pieces (1 for winxp and other for linux)
2. Install winxp on the 120 (1st primary partition).
3. Then install redhat on the 2nd partition.
This is probably one of the easiest setups of having setting up dual boot. Then with your 40 gig, use NTFS filesystem and use that hd for data storage or whatever you desire.
NOTE: When you install linux and it will ask you to use either GRUB or LILO as the boot loader. Read up more about each one to find the adv/disadvantages. To make it short, both run fine
Take it easy and try to be patient. Trust me, in the end it is much more rewarding to you. Like they all say, "no pain, no gain". If you need any help, we're here to help you each step of the way bro.