For future reference it is a BAD very BAD BAD BAD BAD idea to ever give anything of yours away without clearing it out.... especially a computer.... My recommendation:

If you know the person at the very least go through and clear all temp file and blah blah... if you can (I recommend strongly) that you wipe it clean and since it was your mistake offer to reload the OS for them...

That said it's not just your yahoo and email accounts I would be worried about so much as the process of what you should take a look at:

Have you ever done online banking, credit card bill viewing, or any financial data viewing on that laptop?

if so Change all those passwords, if those sites have secret questions to recover your password change those as well.

You said he is accessing (or attempting) to access your work network?

If you aren't the computer admin (just going out on a limb here) inform your IT staff so they can change your password or change it yourself but make sure they know to keep an eye out for whatever you computer name is. If you had VPN setup there I would recommend changing any of that data as well.

Anything that you have that has a password needs to be changed, anything that you have that is connected to monetary funding needs to be closley watched and passwords need to be reset up. Email accounts, Instant Message accounts, you name it you need to go through and change the passwords and secret questions, I may even go to the extreme of changing the PIN #'s on any of your Debit/Credit cards

Just my 2 Cents and I hope you learned your lesson this time around.

next time look for a program that can do multiple low level passes and perhaps rewrite 1's and 0's to the drive..