Imagine your search for a software programme has just yielded multiple results. Which one should you choose? I will be able to choose if someone can tell me:

1.What is the difference is between a dmg, a sit, and a gz file?

2. If it is best to go for a faster speed, which is fastest T1, T3 or higher, DSL?

3. When I have downloaded a software programme, do I hit 'launch' in order to install it on my machine?

4. How can I be sure it does not contain a virus?