Welcome to the Bright Side.
VLC is probably about the best you are going to do for AVI files. There are a number of sites that claim to have a QuickTime plugin for AVI but I have not found one that works well. However, I'm sure someone will come up with a better answer than that.
On to the .sit files. This is the equivalent of the ZIP file in the PC world. You will need to get yourself a FREE copy of Stuffit Expander. I'm sure you have found the place, under the Apple symbol at the top left of the screen, that says "Mac OS X Software". You will find a wealth of software up there. To make it easier for you, here is the link:
Download it and follow the instructions and after that, you will have no problems with .sit/.hqx/.sitx/zip/rar/and a whole host of other compression formats.
To make life a little easier for you, and to keep the desktop reasonably clear, create a new folder on the desktop and name it, "Downloaded Files". Open Safari and go to the preferences: Under the General tab, where it says Download files to: click the drop down arrow, select Other. When the window opens, click on Desktop in the Sidebar and then click once on your downloaded files folder. Click Select. Now everything that you download will go to that folder. Set Stuffit Expander to expand in the same place as the original and you will no longer have to search for your files.
New Mac convert! Bought a $699 Mac mini on 9/5/05. Freedom from Windows is sweet!
Anyways, to my questions...
1) I cannot find a codec for .avi files for Quicktime 7. DivX isn't Tiger/Quicktime 7-friendly, so I'm using a program called VLC right now. Any advice would be appreciated. I was told DivX 5.1.1 would work, but it is .sit file, which leads to my second question...
2) What do I do with .sit files?! A lot of programs I want to use are downloaded as .sit files, and Tiger doesn't recognize them. What do I do to install my program and use these files?
Running: Mac mini, 1.42GHz G4 Processor, 512MB RAM, 80GB Hard Drive, Quicktime 7, OS X Tiger, SuperDrive, AirPort Extreme, Bluetooth