First, adding an OS to a Mac after you've set up a dual-boot between Windows and OS X is very tricky because of Apple's ill-conceived hybrid MBR
setup. If you're not careful, you can end up trashing your data. In particular, you should never, ever
use Windows partitioning utilities to modify the partitions on your computer. It's not clear if you've been doing so, but if you have, I recommend you stop right now and repair whatever damage has been done before it bites you. That will, unfortunately, require a fair amount of expertise.
If you used an OS X or Linux utility to shrink your Windows partition, you've probably avoided that particular pitfall, but it's impossible to say what's going wrong without more technical details. The easiest way to get these is to run the Boot Info Script
from a Linux emergency disc. This will produce a file called RESULTS.txt. Post it here, either between code tags or as a link.