Sounds to me as though you (or your system) have set Windows Movie Maker to be the default Application for opening AVI files.
If you want to change it to something else (Like Media Player, or Elements) do the following,
1. From windows locate one of your movie files.
2. Right click and select Properties (note the Opens with: value)
3. Click the Change button and select the Program of your choice from the resultant list.
4. Click OK
COMPAQ something or other with 500Gb Boot Disk, 1Tb external Firewire Disk, 4Gb Memory and a super duper sound card, 19" TFT Monitor etc etc etc
Sony TRV33e : Sony TRV310e : Canon XM2
Sony Vegas Studio editions to v11
Remember, there is always more to learn than there is to teach.