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There are several reasons you may need to "zip" a large file.  Firstly, the zip program (whichever one you use) will usually reduce the file size considerably, sometimes to a very small ratio of the original size, but more typically 30%.  (Note that some file types, like jpgs, do not compress very much at all: that is because they are already using a compressed format). 

Secondly, you may need to protect a file with a password for the purpose of transferring it via FTP or a file forwarding service.

Thirdly, you may need to send a group of files to someone and would prefer the convenience of bundling the files together into one package.

Some users may also find it useful for purposes of backing up your documents and spreadsheets into one large file.

Zipping a file requires moderately advanced skills but can be managed quite well with a bit of effort.

The first thing you will need is a program to do the compression.  In the IT Department, our current favorite is 7Zip. It is free and available for download from here.   There are tutorials on the site to introduce you to the application.

Windows 7 also has a built in compression utility.  To use it, right-click on the file (or folder, or files) you want to compress and select "Send to" and choose "compressed (zipped) folder".  When it's done, you will see a new zip file in the directory at the location of the original files.



External References

Installation: you may already have 7zip installed on your computer.  If not, here's a quick tutorial on downloading and installing it.

Basic introduction to Zipping and Unzipping in Windows 7.

Fairly decent tutorial on using 7zip.  Slow pace and little "geeky" but useful if you haven't used 7zip before.

Some very good technical descriptions of the options available in 7ZIP.