Does this family member reside in the same house?
Create a new system user account? (If you're using Windows, I think this is possible, on OS X, Linux etc. it certainly is).
Deleting the Firefox installation (and all user files) and installing the latest version (different UA string) could also help, at least in peace-of-mind terms.
But only you know whats on it and where?
Delete everything you stored and worked on.
In IE or whatever browser clear the passwords, autofill, history, etc.
Then run some erasing software that cleans the cluster tips, and the freespace by overwriting with ones and zeros a few times.
Remove all email and mail server settings, as well as any connections, dial ups, and your favourites and the recent documents lists.
Thats about it!
[edited by: jatar_k at 7:01 pm (utc) on April 10, 2006]
hal12b: no, a different house
zCat: Thanks, a different account didn't occur to me, and removing firefox is a good idea
Nitrous: On topic I think as we need to tread carefully with AdSense
delete your profile on that machine too.
Passwords and form info are often stored in the registry and can be retrieved and decrypted with a special tool freely available on the net.
And BCWipe is a good secure erase tool, which allows wiping of free space and specified files.
Holy crap, the more you investigate anything related to computer security, the scarier it gets.
Surely a fresh install is going to be quicker than messing around trying to cover your tracks?
Not that it would help if the recipient really wanted to get to your data, of course...
Going to say the same thing. Just format it. Its way easier than dealing with the details.
format it and let they install new OS themselves.
you can be sure they find time :)