Installing Zorin on existing Ubuntu partition

hi, i have windows xp, ubuntu 10.4 and ubuntu 11.10 installed on my computer (3 main partitions and multiboot). everything works just fine.
i tried to install zorin 5 lite over my ubuntu 11.10 without resizing. i selected /sda7 and click forward but got error message 'no boot system selected'(? do not remember exactly, it was yesterday night
it there any detailed installation manual available, please? this one is very general... http://zorin-os.com/installguide.html

error massage is:
'no root system defined.
please correct this from the paritioning menu.'

partitions look like (dev/sda):
sda2 fat32
sda1 ntfs
sda5 ext4
sda7 ext4
sda8 swap
sda6 swap

hi, i install zorin 5 lite. the only problem i still have is that i don't know how change order/default os in boot/startup. i want ubuntu be the first option, not zorin. i tried to install startup-manager (http://techhamlet.com/2010/02/edit-grub-boot-loader/), but somehow i don't see it. any help please?

My hard drive is completely empty and its saying that I don't have enough free space to create a partition when I choose erase and install. And when I go to partition it manually I simply don't know how. If you guys could help me out that would be appreciated. Screenshots are helpful seeing as I don't know much of the lingo.

Also when I do create a partition is comes back with that error message in that first reply and I simply don't know what to do.

HELP!

Here is a complete guide to partitioning:

http://www.dedoimedo.com/computers/gparted.html

With regards to the "not enough space message", try the gparted partitioning tool first and see if that helps you!!.

You can download the tool here:

http://gparted.sourceforge.net/download.php

Do not dual post!!!!!, your other post has been deleted!!!.

Regards Wolfman