ManyCam and SplitCam are two free software for Windows XP / Vista / 7 (ManyCam is also available for Mac) that allows to use videos as source for own webcam in an easily way. Specifically you can create playlists, loop mode available, various formats supported (AVI, WMV, MPEG, FLV, etc…)
Once the program is running you just need to change settings in video preferences of your instant messaging (MSN, Skype etc…) by selecting ManyCam or SplitCam from the list of devices; same thing for the cam chat via web (modify Adobe Flash preferences as in the image below)