Do not need a separate version of windows
You just need to set up another account her files will be stored in C: \users\her account name\ documents... pictures...music etc.
It is possible also to move the users folder for each account to a different partition or drive if you require.
If you do not want her to access your filesthen just make sure she is set up as a guest rather than admin account.
Instruction on making a second account click here