a home PC? Either way the password expiry is controlled by the policy setting, this is either Domain Policy (network) or Local Security Policy (home PC).
If it is a home PC then go to Start - All Programs - Administrative Tools - Local Security Policy - Local Security Settings - Account Policies - Password Policy, and set the expiry to 0 (I think)
If you have a networked PC then unless you are the network administrator it is unlikely you will have the authority to change it.
Also if you are on a home PC you can remove then requirement for a password in the Accounts & Users in the control panel.