As Jelson said, get a backup.

After you are done with backup, get a windows XP CD. boot your system with Windows CD and install windows XP on it. It will need a full format of your HDD. If you got just one partition, divide into two at-least. I normally go for 3 or 4 partitions.

Now install XP on C partition. After you are finished with XP, just install Windows 7 on any drive like D or E either by booting using your windows 7 DVD or by directly running the setup.exe from the DVD from inside Windows XP.

This is method one in the tutorial link I gave in my previous reply. Hope it doesnt bring you back to square one.