Depending what applications you've got installed, you could shrink your C partition to 100GB leaving 800GB free for your data partition.
Unfortunately, Windows 7's disk management utility is very basic and will probably only let you shrink the partition by half, leaving you with around 450GB for both your Windows and Data partition when you create it.
If you have a set of recovery disks to get you back to factory restore should you need to, you could delete the OEM recovery partion, extend your C partition to take up the space released, and then shrink it to create space for your data partition.
Just remember that recovering your system from DVDs rather than a recovery partition does take a lot longer.
You might want to consider downloading the free version of
Partition Wizard to partition your drive as it will let you create a partition size you want.
You may also want to consider a clean install of Windows 7 using the OEM product key on your computer as a factory restore will wipe any partitions you have created and any data you haven't backed up first.
With a clean install you're not affecting anything other than your Windows partition when you carry it out.
Once you've got everything set up how you want create a complete system image backup and restore from that should the need arise.
Just be careful when you're changing the default location of your User folders and Public folders as a broken junction point or symbolic link will cause you problems.
In case you didn't know, Windows 7 uses these links to tell legacy software where the default save location is. For example, software written for Windows XP would look for My Documents, all the junction point does is point it to the Users\Documents folder in Windows 7, but if the junction point can't be seen or has been deleted, then the legacy software gets mightily confused.