As everyone has stated it is not a good idea.
what I do is create another partition and on every install, choose customize, and just replace the letter C with a E
You can however, change your "users" folder, but you need to do this immediately after install.
All you have to do is change your environmental variables. Click on Control panel > System, click Advanced System settings and click the advanced tab, click the Environmental settings and change the USERPROFILE path.