If you have installed Windows 7 and allowed it to automatically partition your hard drive during installation, click the Start Menu, type disk management and hit enter. It will display as the first partition on your Windows 7 disk.
Alternatively, run diskpart.exe and:
1. Type list disk and take note of the disk number where Windows 7 is installed.
2. Type select disk #, where # is the disk number you noted before.
3. Type list partition.
Some more info here:
How to Avoid 200MB Hidden System Partition From Been Created During Windows 7 Installation » My Digital Life
Of note, also Bitlocker. Yet another Windows 7 functionality that will create a partition (much larger, 1.5GB) that is irrelevant to Windows XP.