One of Windows' lasting problems is the mixing of program and data files on a single partition by default. Changing the default user directory to another partition will help keep data files isolated from the more volatile program files. This setup also allows reinstallation of the operating system without destroying data files, and it can increase system performance if the program files and data files are on different hard drives.

Windows keeps the default user directory setting in the registry under the key, HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList. Using an account with admin privileges, you can change the path under that key, called "Profiles Directory".

For best performance you'll want to use two separate hard drives, one the operating system and programs, and the other for user directories and data storage. If you don't have separate drives you can still use logical partitions on a single drive to make it much easier to reinstall your operating system.

I recommend reinstalling the operating system at least once every two years, so keeping data files separate is important or else you'll need to backup your data files before upgrading.