To increase security, Windows may ask you to periodically change your password. Personally, I hate this "feature". I have enough passwords to remember and a hard enough time remembering them already. I don't need more passwords to try and remember.

If your PC is part of a network that has these types of rules set by a network administrator, then there is nothing you can do to keep Windows from expiring your password. But if you have control of your machine, it's pretty easy to set things up so that you don't have to keep changing your password.

Go to the Start menu and enter lusrmgr.msc in the Search box if you are in Vista, for XP users, go to Start -> Run and enter lusrmgr.msc. Select the Users tab on the left, and then select your user from the right side of the screen by doubling clicking on it. This will open the properties dialog for your user. In the General tab there is a box labeled "Password never expires". If you check this box, you won't need to keep changing your password, you'll be able to keep the same password forever.