nearly all distros are installable.
some popular ones are :-
Ubuntu
Xandros
Fedora Core
Kubuntu
Gentoo
Edubuntu
Mandriva
Debian
Linspire
Lycoris
Red Hat
Turbolinux
for more distros, chk out *www.distrowatch.com
install linux on a different drive. install GRUB loader, which creates a dual boot.