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Codecs are specification files that include various information and code to allow playback of various video and audio file types – and from time to time they get corrupted.
Corrupt codecs are bad and could well be the cause of your Windows Movie Maker issues.
To reinstall these, you first need to remove them. Go to Start → Control Panel → Sounds and Audio Devices and on the Hardware tab select Video Codecs and click Properties. On the next window, go to the Properties tab and one by one remove the codecs.
The next step is to reinstall the codecs. Go to www.codecguide.com and download the K-Lite Codec Pack. You have a choice of Basic, Standard or Full– I would recommend any choice other than Basic. You may be experiencing problems editing video clips recorded on a camera phone for instance– many mobile devices record in unusual formats, so the more codecs you have to decode them, the better.