How can I change the "protected card" so that I can once again take pictures - and how can I keep this from happening again?

Hope you are trying to do this after downloading the software you have got with the camera. Hope the memory card is not protected. You can check these once and try again.

Probably one of two solutions.
First, the memory card should have all the pictures intact. If you are using the camera to review, you may need to just keep going back until the memory card photos show.
Another possibility is that the internal memory supersedes viewing the card memories. You may have to run a USB cable to your computer, download the internal memory photos, and then you can possibly see (on the camera) the memory card photos. It might be a good idea to download those photos to your computer too, if the computer has a memory card slot. Otherwise you will have to go through them and download via the USB again. Once you know the photos are saved, you can either change cards with a new one , or clear the old card and start again.
Good Luck

I don't think the XD card has a lock switch (some do) but what I fully expect is that the memory card requires your to format it to the camera you are using it in. You didn't tell me what camera make or model But you did say you have a manual, look for formatting or it may be called initialize. Formatting clean the surface of the memory card and makes it ready to accept new pictures again.

Possible that the card has the small lever on it pulled down so that it protects from over wrting.
So remove the card and look on the side for aa small switch showing a direction to protect, move it off and it will allow the card to write again.

SD and SDHC cards have a slide switch along one edge. The position farthest from the contacts locks the card, protecting it from writes. The position nearest the contacts unlocks the card. If the switch is already in the proper position, slide it fully the other way and then back again.

Make sure the slide switch on the side of the card is unlocked. Your camera may just want you to format the card in the camera. Download any pics from it first using a card reader if necessary.

There is a minuscule tab on the side of the card, towards the "handle"-end of the card. Slide this tab to unlock the card and allow write and delete once again.

This is a general problem, not specific to your camera.

Inside your camera, there's a memory card. In your case an SD card. This card has a slider that can be toggled between two positions. Your card's slider is in the "protected" position, and means you can't write to the card. This position is used when you want to be sure you don't overwrite important files (in your case pictures).

The sliders on these cards sometimes change positions when you jam them into a card reader - which might be what happened here?

Simply take the card out, find the slider, and slide it over to the other position. Insert it into the camera, taking care to not slide it into the protected position again, and you should be all set to take pictures!

It's the memory card. SD cards have a slide switch along one edge. The position farthest from the contacts locks the card, protecting it from writes. The position nearest the contacts unlocks the card. If the switch is already in the proper position, slide it fully the other way and then back again.

i know it's not solving your problem, but at least it stop from happening again... You can protect images/video clips from being accidentally erased using the protect function in the Playback menu... see also: http://www.fixya.com/BrowseManual.aspx?prdid=218406&mpgid=843814&hlt=erase