change ownership of a computer

I just bought a used computer and I want to remove all evidence of the new owner with out formatting the HD. Its a Sony Vaio with win XP madia edition.
How can I do this?

CAUTION: *make sure* you have a new login for you AND
the admin login is also available (& you have the password) with admin privileges.

login as ADMIN

then:
go to \Documents and Settings
find the directory for user to be obliterated and click into it

find the Documents folder and click into it
select all and delete

this will delete all documents saved by that user (normally anyway)

there are two other folders to clobber
Application Data and Local Settings

now the data and settings are gone, use the CP-> User Accounts,
and delete that user w/o saving files.

this will dump anything that was NOT EXPLICITLY saved elsewhere

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Welcome, mate!

Cheers