You need to create a new account, then copy your old user profile over to the new account, when you are happy with the new user account delete the old one.
Scroll down to "copying your user profile"
This article is for XP, but the instructions are very similar for Vista/W7
There may be other documents you need to copy and paste from the old user documents folders, be sure you have everything copied before you delete the old account.
See "Copy files to new user profile:" under Step B
You can change the account name, but doing this will Not change the name of the user account folder, it cannot be changed, you have to create a new user account with the name you need.