The per-user profile folder in Windows ( C:\Documents and Settings\[User] in XP and C:\Users\[User] in Vista ) cannot be changed.
You can however create a new user account with a new name, then move any files from the old user folder to the new folder and delete the old user account.
You can do this with User Accounts in Control Panel.
Note: You will lose any NTFS Encrypted Files created with Windows Vista Business or higher edition of Vista this way unless you unencrypt them first.
You would also lose any application preferences.
Though since you just got this computer from a friend you probably want a new user account with your own preferences anyway.