Install ubuntu when there is Red hat already installed

[mythripk]

Hi All,
In my PC i have Red hat installed and now in want to install ubuntu , I want both to co-exist.
Red hat is there in /dev/sda1 and /dev/sda2 , sda3 is the swap space. There are no mount points defined for either sda1 or 2. Hard disk space is 160GB and Red hat partition along with swap has taken up 20GB space
My problem :
1. I tried to create new primary partition with mount point as root with 15GB space it showed rest of the free space as unusable.But still i continued assuming that it can be partitioned later.But i did not see ubuntu as an option on boot up , but when added in the grub of redhat it showed the option but i said failed beacuse of some error (ext2fs does not match with partition 0x83) :(
2. I tried to create a new logical partition with mount point defined as root and it installed but again same problem seen above.

I am not sure as to how to go about it and am naivete with multiple linux installation on same machine.I need to do it asap.
Can anyone please help?:confused:

Thanks and regards