1 An OS image is your entire Operating System backed up but in a light compressed form, in simple terms a picture taken of your entire system, it can be put on a disc or a partition or an external hard drive, so if your computer gets a virus you cant shift or you delete a key registry or system file and your computer is not working correctly all you do is locate your backed up image from where ever and select restore, your hard drive will be wiped clean and all the data on you image back up put on, in its place. you will have a good working computer in about 20mins.
You can do this as many times as you want, It save the hassle of a reformat and then having to reinstall all your software again.
2 What codecs are you using and what system are you using XP?