Sorry, but that's not a lot of information to go on.  Here's a list of helpful information you can provide to aid in the diagnostic process.

Having directed you there, I'll still ask this:  did you install the K-Lite codec pack?  If yes, you're hosed.  No one should ever use that piece of garbage.  You are doomed to reinstalling EVERY piece of software on your machine, and that includes the the operating system.

And to quote from Mylenium -- a mainstay contributor to this forum -- "Divx is evil."  From that, I think you can guess what you should NOT be using.

themanofvideo wrote:

Are you sure total reinstallation is the only alternative?

Pretty much. 

You can confirm by doing TWO searches in this forum: one using the term Klite or K-Lite and one for AVI.  The first search will reveal others' experiences with K-Lite, and the second should reveal Todd Kopriva's recent post on AVI woes like yours.  You'll find the search window in the upper right the page.

Looking over your specs, I'd also double check AE 10's (or 10.5's) System Requirements.  I'm not certain if Vista 64-bit is up to snuff.  You might need Win 7 for best results.

Now here's something I AM certain about: you're way low on memory for AE work.  4 gigs is almost nothing.  There's no way you can use multiprocessing.  I would say that 8 gigs is the minimum to run AE effectively.  16 gigs lets you use a couple of cores for multiprocessing.  24 gigs is good for multiprocessing.  More memory than that lets you leave other applications open with no worries.

I don't, I'm a Quicktime guy.  I use it for everything, even on my Windows box at home.  AVI is Terra Incognita in my world.

But don't overlook that second search I recommended: somebody recently had AVI problems like yours, and Todd had the answer.