Important:
If you have this MacBook registered with iTunes, iCloud (etc.), be sure that you SIGN YOURSELF OUT of these accounts before you do anything else.
PRINT THIS OUT FOR REFERENCE.
What I would do (this presumes that it runs ok with the existing OS install):
1. Go to the Users & Groups pref pane.
2. Create a NEW account with administrative privileges.
3. Give it a simple name and password for the moment (it may be removed later)
4. Next, LOG OUT of your account, and log into the new (temp) account
5. When you get there, click the lock icon and enter the (new) password.
6. Now, RIGHT CLICK on your old account and choose "advanced options" (humor me)
7. DON'T CHANGE ANYTHING -- you're "just looking".
8. What you're looking for is the account ID -- it should be "501". This is normally "the main account" ID#. We just want to confirm this before you delete it. Click cancel to return to Users & Groups.
9. If it is the 501 account, do this next:
a. Click on your old account ONE TIME to select it
b. Down at the bottom click the "-" sign
c. You should now see a dialog asking "are you sure you want to delete the account "xxx"?"
d. Yes, we're going to delete your account. Choose "delete the home folder" and then click "Delete User".
e. That should nuke your existing home folder, leaving the "501" space "available"
10. Now, click the "+" sign and add a new account for your wife.
11. Consult with her as to what she wants for a username.
12. Give her a temporary password -- she can change that on her own later on.
13. Now, log out of the temp account and log into your wife's account.
At this point, you can either delete the temp account (same way you deleted your old account in step 9 above.
or...
You can leave the barebones "temp" account there. If you need to do maintenance on the MacBook, your wife can log out of her regular account and you can log into the temp account and "do what you need to do"...