Heres how to do it in Windows 7 & XP.
Note: This uses Local Security Policy which is not available in Home versions of Windows.
These are local prefs on machines not on a domain.
Local Security Policy opens.
Navigate to Account Policies \ Password Policy and double-hit Maximum password age.
If you were set it to zero, the password would never expire.
Changing Password Age in XP ProfessionalWhile easy in Windows 7, XP gets a bit more tricky.
First go to Start \ Run and throw in mmc and Enter.
Under Group Policy Object verify it says Local Computer and click Finish.
Then as before double-click Maximum password age.
Then change it to whatever you want it to be.
You should have users change their password regularly and encourage them to use strong passwords as well.