There are a few things you might want to understand before installing Ubuntu. First, Windows 7 uses NTFS, where as Ubuntu uses ext4 as default for their file system. What this means is that, although Ubuntu can see and access Win7's file system, the reverse might NOT be true.. Thus, if you are thinking of sharing files between the two OSes, just save it in a NTFS drive used by your windows to be able to access it via both Windows and Ubuntu.
More importantly, this brings me to my second point. Their is no practical advantage of installing both OS on the same hard drive. Further, if you wanna choose to use the Windows Boot-Loader: Winload (The black screen during starting your computer that lets you choose between OSes) instead of Grub2 (default boot-loader for Ubuntu), it is highly recommended that you choose another drive, as separate boot loaders on separate systems will ensure that they do not overwrite one another in the master boot record(MBR).
I can give you tons of more reasons why installing two different OSes on the same partition is a bad idea. I would hence recommend installing Ubuntu on another drive.
Now, if you are having problems partitioning the drive, please note that windows 7 does not allow more than 4 drives. Thus, if you are already using 4 different partitions, I'd suggest that you should merge any two of them together and create a separate drive for Ubuntu (or clear up a drive completely, and replace its usage for Ubuntu).
Last but not the least, here's a few links to help you with disk management in win7:
(1) http://www.support.com/blog/post/resize-your-hard-drive-partition-without-losing-data?geo=IN
(2) http://www.howtogeek.com/howto/windows-vista/resize-a-partition-for-free-in-windows-vista/