My guess is that on the first attempt you tried installing to your Win NTFS partition /dev/sda1.
Good luck fixing that.

For your fstab entry you'll want something like
Make the /mnt/data (or wherever you want it) with rw perms for your user(s)

No I didn't. I made a new 50GB ext3 partition and installed Linux to that. The Windows partition is still there. I just can't boot into Windows. I've got other errors now with GRUB and posted a new thread about them.

What if you used FAT32? Both Windows and Linux fully support them.

Not sure how the permissions would work on it though...

For a Windows FS manager that supports Linux filesystems, I use Ext2Fsd. It fully supports ext2, works well with ext3, and is experimental with ext4. It can also be used to mount a FAT32 flash drive to a drive letter when Windows fails to auto-mount it, if that's a problem on your machine (it sometimes happens on mine).

Hope this helps you with what you're looking for!

I hope you get your install fixed.

What I do on my dual-boot systems is:

windows:
once install is done, move the documents and other folders I want to point to folders on the shared data partition. this can be done in the right-click menu of specail windows folder (documents, music, etc...). I don't move the settings folders. Makes life simplier in the event of a reinstall.

linux:
use symlinks in the default home folder to the folders I want on the shared data partition. I leave application settings in the /home/user folder, same reason as with windows. keeps the app settings separate from my data. Handy if distro hopping or reinstalling after a bad experiment.

and back up back up back up. especially when messing with linking file systems.

I dropped back to Mint 13 and it installed and dual-boots fine. I've got my system working like I want. I really think my hard drive is about to fail, so I backed up all of my stuff. Probably why I had problems with installing GRUB.

So, basically what I did was this:
Install Windows 7 on a 50 GB NTFS partition. Just Windows itself took up about 20 GB!
Install Linux on a 50 GB ext3/4 partition. The Windows Ext2Fsd currently has better support for ext3 than ext4, so I used ext3.
Make a NTFS data partition that spans the rest of the disk and put a single <username> folder on that partition and create subfolders in it such as Pictures, Documents, and Music.
Make symlinks in your Linux <username> folder to the subfolders on the data partition.
Change the location of the Libraries files to the subfolders on the data partition.

I might also try to symlink my Windows home folder to the data partition, so I get more transparent access. It's so much harder to symlink in Windows, though.

If you really want to get fancy you can do an unattended install of windows and have it replace /users with /home