Safe pain in the butt way:
Make a new account, copy (pros/cons of each) data files to shared portion of filesystem (called Shared) in /Users/Shared/, and reconfigure your apps, and settings and everything by hand.
Scary way:
Make a new admin account. Log in as root and remove the default home directory they give you, and copy your old home directory to a new directory named exactly after your username (the short one, not the long one).
This way you keep your desktop image and and some other stuff.
Either way, I'd say keep the old account around, in case something is haywire.