you could use partition magic to shrink the windows partition before installing red hat...
but really IMHO the best thing for you to do would be to install mandrake linux instead of red hat... mandrake's super-friendly installer comes with a built-in partition re-sizer, so you wouldn't need to do anything special before the installation (besides a defrag and a scandisk, of course)...
the other reason why i would suggest mandrake is because red hat 9.0 has only a few more months of support left... i'm sure you heard by now about the change from red hat linux to fedora core, right?
http://fedora.redhat.com/
http://www.mandrakelinux.com/en-us/
personally, i wouldn't install something that i know won't be getting maintained after a few months...