I use Yahoo mail and when I receive an email with a "doc" file attached I can´t open it. What program do I not have on my computer that would let me open "doc" files?

A ".doc" file name extension is normally associated with a Word document

You can associate it with wordpad, save to your hard drive and then right click on the .doc file and choose open with... and then browse to wordpad

Usually, c:\program files\windows nt\accessories and then wordpad.exe

If not at the location, open wordpad in accessories tab in start menu and then choose file, open and then browse to your downloaded file

Openoffice.org is an excellent substitute for MS Office. It is free and available for Windows, Mac< Linux and others.