Remove All Traces Of Password To Connect To Internet

This may sound strange (or not), but I wiped the hard drive of an old laptop pc and then reinstalled the operating system and then used my internet connection to get all the updates for the pc before donating it. However, I would like to make sure that there is no way that someone can connect to my guest wireless account in the future (short of having to change passwords on my router).

I had no problem disconnecting the pc from the connection and removing the information, but when I click the internet icon and it finds the available networks and I click on the guest account it is still automatically connecting without asking for the password. Is there someway to get the darn pc to forget that password?

It is an old laptop running windows XP Professional.

Thanks So Very Much.

 

The XP will be donated to someone who won't live near you.
When that person clicks available networks, they will not see yours unless they are in your home or sitting in your yard
Not seeing your current network, there is no way they would be able to connect.