XP Upgrade Path
You must have a copy of Windows XP on your computer to do a clean install of Windows 7. This is the upgrade path as you should make a partition and than install Windows 7 on that partition. Make sure the partition is at least 40 gigs as if you want to add programs and documents, pics, etc you will need the space. When you install Windows 7 use the custom install and create the partition using the free space on your hard drive. Remember do not use the space allocated to Windows XP and this will create a dual boot with XP and 7 allowing you to select either when booting.
The advantage of doing this is you can than transfer your personal items right from drive to drive. Remember that in doing this the Master Boot Record is going to be on the first Drive so if you decide to delete or uninstall one drive you will have to recreate the MBR or you will not be able to boot.
There are other ways posted on the internet but remember this if for some reason you get errors and have to transmit info to Microsoft for help you may be caught violating their EULA (License) and your copy of Windows 7 could than be invalidated.