Download and install Mediainfo.

Post the media properties on the forum or look at the video and audio sections from the Mediainfo report to determine which individual codec(s) you need.

AVI is a wrapper, you need to determine what type of audio and video you have in order to know which codecs to load.

Vegas only uses two installable codec systems.

Video for Windows files, aka AVI. So this might be the source of your AVI problem. You might be missing a codec for that. Vegas does not install anything for Video for Windows files itself.

Quicktime, aka MOV. Vegas does special case some MOV files and handle those directly.

Vegas handles everything else directly itself. Either it supports it or there is nothing you can install to get Vegas to support it.

Vegas is supposed to support AVCHD natively.

Is it an m2ts file?

No complaints or warnings about the file from Vegas?

When you drop it on the timeline, you get a video track on the timeline? Do the thumbnails for the clip show anything.

What does mediainfo or some other dumper show for that file?

In Vegas, right click the media file in the project media window, select properties and what does the general tab in that dialog list? In particular what plug-in is listed as being used.

"m2ts file?"

MPEG2 Transport Stream, AKA HDV.

Mediainfo report is as follows:

AVI(OpenDML): 7.17GiB, 12mn 15s
1 video stream: CineForm
1 audio stream: PCM

First video stream
82.1 Mbps, 1920x1080(16:9), at 29.970 fps, CineForm
First audio stream
1 536 Kbps, 48.0 KHz, 16 bits, 2 channels, PCM (Little / Signed)

When I try to drag the above avi file to the timeline I get the NO symbol and it won't drop. It won't do anything.
AVCHD from a Canon 5D drops OK but the preview window is solid white and doesn't play any video though I can see stills on the timeline. Audio plays OK.

the avi files were already used in a major project of mine on another machine, an i5, where everything worked just great. This is a gamers cybertron with two GPUs I bought at Tigerdirect.

I'm sure something is missing here but who knows what it is?

"82.1 Mbps, 1920x1080(16:9), at 29.970 fps, CineForm"

I do believe Cineform is a proprietary codec which you must install.

For the AVI, You need to install Cineform. It is a proprietary codec. You can get that free by installing the GoPro Studio software (the free version). GoPro bought Cineform a while back.

Canon DSLRs do not generate AVCHD files. They generate MOV files using the AVC codec. Sematics maybe, but when you have a problem we need to be specific.

Vegas should not have a problem with AVC MOV files and it does not need quicktime to use those. In the media properties dialog you should see Vegas list compoundplug.dll as the plug-in for reading the 5D file.

If thumbnails exist in the timeline event then obviously the decode is working at some level. If this is an existing project, try importing into a blank project.

Does the 5d file play in the trimmer?

...so I loaded the free GoPro Studio on my new computer and..and.. and...
it works. avchd..(excuse me) I mean MOV files drop and play too. The feeling I have since I don't know for sure is some missing pieces of the puzzle were provided by the GoPro software. Although I'd sort of like to know what that might have been I'm happy with the fact it plays without any interruption, no hesitation at full resolution.

Tanks guys. I'm a happy man.

The cineform codec is now owned by gopro and is on the gopro software disk which was why you needed to install.

Oh and one more thing...

new i7 with two GPU with 32 GB super Wham-O-Dyne computer
Where have you been all my video editing life?
Dis is great!