1) Go to the Directory in the top menu. The first choice is download Linux. Although, one does not just download Linux, one downloads a distribution of Linux, depending on what your computer is used for, the hardware it contains and what your particular needs are. For new users I usually recommend the Linux Mint distribution, because, it is the one that is easiest to set up, in my opinion. Usually we recommend downloading a Live CD, so you can experience using Linux before doing an install. Almost every distribution offers a Live CD, or, a Live USB in case your using a netbook or other type of computer that does not have a CD player attached.
2) There are many differences between Linux and Windows Vista. The most valuable difference is the freedom to use, share and modify it to your own specifications. You can't do that with Windows, it is closed, Linux is Open. If you would like to understand what Linux is about, a short video from our 20th Anniversary tells the story pretty well. Watch here:

http://www.youtube.com/watch?v=5ocq6_3-nEw

3) As I explained in "1)", I usually recommend Linux Mint. As far as being closest to Windows, most distros are set up to somewhat look like Windows. Those using the KDE window manager come closest, although, the present Gnome window manager used in Linux Mint is also similar to the Windows desktop. It doesn't take very much to get used to using either.
4) Internet Explorer and Microsoft Office are not compatible with Linux. Microsoft does not put out versions that run on Linux based operating system. Firefox and Google Chrome are much better replacements for Internet Explorer, and, are much more secure. Internet Explorer has had many vulnerabilities over the years, and since Microsoft doesn't update their products in a timely manner, it is still true today. OpenOffice, or, the more recent LibreOffice are good replacements for Microsoft Office. They work well for me and many others, but, since Microsoft doesn't align itself to the Open Standards, sometimes converting a Windows Office created file with OpenOffice or LibreOffice, can create problems. I convert Microsoft Office files all the time and have not encountered those problems. It all depends on how you use Microsoft Office, and, how many of the advanced commands you embed in your documents/spreadsheets.

There is also a web site called DistroWatch, where you can look at ALL the Linux distros available, if you interested. Link here:
http://distrowatch.com/index.php?dataspan=4

If you need certain specifications, or would like to tell us what kind of computer you are using, or would like any other questions answered, please just ask. That's what we are here for.