There is a way; it's called 'dual booting'. Unlike Windows XP, with Windows Vista and Windows 7 it's more complicated to install Linux with Windows. Broadly speaking, this is what you need to do if you already have Windows Vista or 7 installed to your HDD with a single partition holding Windows Vista or 7:
1. Reduce the size of the existing partition.
a) Defragment the Windows partition.
b) Use Windows' Disk Management to shrink the partition (see http://www.howtogeek.com/howto/windows- ... ows-vista/
). The problem with this method is that Windows will only shrink the partition until it reaches the MFT (Master File Table), which means there will be a lot of free space in the Windows partition that you may have wanted to use in a Linux partition.
c) If you cannot shrink the Windows partition to the size you want, then you need to move the MFT (see http://www.howtogeek.com/howto/windows- ... -problems/
). Basically you need to download a shareware ('trialware') tool that will move the MFT and defragment the Windows partition to a size you select.
Do NOT use GParted to reduce the size of the existing Windows partition, or you will screw up your Windows installation.
2. Create the new partitions for Linux
Boot a LiveCD/DVD which has GParted on it, and run GParted to create and format partitions (for example /, /boot, /home and swap) in the free space you created in Step 1 above. SystemRescueCD is one good LiveCD to use for running GParted.
3. Install Linux and bootloader
Boot Windows Vista/7 and download from http://neosmart.net/dl.php?id=1
the freeware tool EasyBCD. N.B. You will need to download EasyBCD beta version 2 if you want to install a Linux distribution that uses Grub2. If you're going to install a Linux distribution that uses Grub1 (e.g. SL 5.1-r1) then the current version of EasyBCD should work. Follow the instructions on the EasyBCD site for how to install Linux (http://neosmart.net/wiki/display/EBCD/Linux
Hope I haven't forgotten anything.
If you only have an 80 Gb HDD then you are going to struggle. I've read that Windows 7 should work with only 50 Gb, but that would only leave 30 Gb for SL. You could fit SL in that, but it doesn't give you much room.
EDIT (May 9, 2010 15:49 BST): I have created an article in the SL Wiki about how to configure a PC with Windows 7 or Windows Vista already installed to dual boot with Sabayon Linux: Dual Boot Sabayon Linux and Windows