I'm inexperienced with Linux, although I've been playing around with various desktops for the last three years, but not really goin much further than installing it, playing with desktop settings, then reinstalling another distro.
What you aim to achieve should be pretty simple dependin on which distro you use (I've always found Mandrake, now Mandriva simplest to install).
Step 1: Fully backup Windows!!! If you don't, sods law dictates you will lose all you precious data.
Step 2: Download and burn ISO images to CD/DVD, or purchase (if you want official support) your Linux distribution of choice (I currently use open-source version (free, but no official support) of Suse 10 OSS 64-bit).
Step 3: Insert Disc 1 of your burnt CDs/DVD.
Step 4: Follow onscreen instructions. When partitioning instructions appear, your distro should detect you have Windows installed on one of your drives, and ask if you want to use any free available space, or format all your space. Choose to install Linux on free available space, sleecting the relevant drive (in your case the 40GB one), and you should see a warning, if your 40Gb drive has any data, on it that you will lose it all. This is where, having backed up all your Windows data, your heart will not race as fast!
It should all be pretty much automated, so good luck, and let us know how you get on!