Don't know where it is stored on the top of my head, but removing it is very easy. Select the account you want to remove. On the login screen click on the down arrow on the lower-right.
Click on the 'Forget me' link. Done.
EDIT: Too slow
.
And yes, if you're about to pass the machine on to someone else just format it completely. Use the restore CDs/partition if it came with any, or just use the Windows CD to format it and do a fresh installation. There is no doubt there is more private data buried in the system than just those login details, both the old and the new owner benefit from a fresh OS installation.