Step-by-step beginner's guide to installing Ubuntu 11.10

You'd have to do the something else option and then manually remove, then create the partitions required for Linux in place of C Partition.

The following steps will work (but make a backup of your data partition first!):
1. Choose something else option.
2. From the partitioner, remove the first and second partitions (Windows C and System Reserved). Do not touch the third, data partition at all.
3. Create 30GB Ext4 partition for root.
4. Create xxGB partition 1.5x your RAM for SWAP.
5. Create remainder of space for Ext4 partition for /home
6. Continue installation as before.

You'll be advised to edit fstab to automount "data" when booting (unless you want to manually mount it every time you load Linux), as well as ensure "ntfs-progs" is installed. If you decide to move to Linux full-time you can just create a Ext4 partition and mount it as /data if you prefer.

Thank you, I'm glad my experiences can be put to use in helping others.

@ozandernek No, as long as the hard drive doesn't already have any operating systems on it, it'll be fine. But verify the hard drive in question is clear of any important files or operating system files before proceeding.

The "this computer currently has no operating system" is normally encountered when installing Linux to a second hard drive, or a brand new, unused hard drive. You won't see this if you're installing Linux to the same physical hard drive used for Windows, OS X or other operating systems.

As always, making backups first will give you a means to reverse any changes should something go wrong.