PartedMagic, GPartedLiveCD, or a partitioner on a linux live-cd should tell you what you have for partitions and create partitions for you. Open a terminal and use the command, "fdisk -l" as "su" or "root" to read the partition table.
You may be running up against the four primary partition limit if the Windows installation has a hidden recovery partition, leaving only two more. "fdisk -l" should see it as a additional partition formatted to Windows.
The way around the limit is to format the space you should use for linux as an extended partition, with logical partitions within, formatted for linux.
On Vista, it is advised to use the Windows partitioning tool to change the size of the Windows partition if Windows is to retain its boot integrity. This may well be the case with Windows 7 as well.