Ill update this topic as we go through it, and put here any program which is needed in order to decode diferent files or open them, for question or request for a specific program for decoding feel free to post here a request and if possible ill post it here, make sure u mantion the 3 last letter after the point (e.g. *.zip, *.ppt, *.txt)

Zip files
win zip

WinRAR is also a good program that will compress files even better than WinZip.

It makes files that end with *.rar but it also opens zip files.

If you don't have WinRAR you can use google to search for it

Spoiler:

Other programs that you might need but might already have:
(I am assuming you are all using some kind of Windows)

Acrobat Reader for opening PDF documents (pdf) Download it from here if you dont have it: http://get.adobe.com/reader/

Office 97 - 2007 for opening Word documents (doc) Excel spreadsheets (xls) and Powerpoint presentations (ppt and pps)

You need Office 2007 to open the new office files which are the same as in previous versions of Office but have an x at the end (docx, xlsx, pptx, ppsx) or have Office 2003 with the Office 2007 file reader.

You can also use OpenOffice instead of Microsoft Office if you want.
Its free and can be dowloaded here: http://download.openoffice.org/