To get rid of a mac or PC, you need to deauthorise all computers, then authorise the computers you have again.
You can only deauthorise all computers once a year.
1. Launch iTunes on a Mac or PC,
2. Click Store -> View Account and enter your login details if required.
3. Look under summary, there is a computer authorisation label, and an option to deautnorise all computers. If the option isn't therer, you'll have to wait a year from the last time you did this.
4. Choose that option and follow any prompts.
To authorise a computer
1. Launch iTunes on the computer to be authorised.
2. Click Store -> Authorise This Computer and follow the prompts.
One final word, the 90 day minimum for associating a device to an iTunes account only applies if the account is different from the previous account. If you are associating a device to the same account as the last time, the 90 day minimum does not apply.