Upgrade Gotcha
Once you've downloaded Acrobat 6, double-click on the file and the installation should go smoothly. However the installation isn't quite as smooth if you have an earlier version of Acrobat Reader already installed (and most of you will). You should uninstall any older versions of Acrobat before installing Reader v6.
The v6 installation does NOT remove earlier versions of the reader software nor are you given any option to uninstall then during the v6 setup. This is a strange choice for Adobe to make. It was probably done so developers could have multiple Reader versions present for compatibility testing. However most users don't need that since the latest Reader will read all PDF files, regardless of version.
The result is that you'll have both Reader v6 and an earlier version (probably v5 or v4) on the same computer. That's not a serious problem until you later decide to do some tidying up. You go to Control Panel | Add / Remove Programs and look down the list of installed programs. You'll see that there's both Acrobat Reader v6 and an earlier version. In the spirit of computer cleanliness you uninstall the older version. Good choice but then you can't read any PDF files!
Reason? when you uninstalled the previous Reader, it removed the file association that tells Windows to use the Reader with any .PDF files.
The solution is to reinstall Reader v6, naturally enough. But it should not be necessary in the first place, the v6 installer should detect it's ancestor and offer to remove it.