What are you trying to do or rather why do you need the MS Windows emulator? If you can share with us what you are trying to do or what software you are looking for it will be more helpful in assisting you.
In any case, from what it sounds check the following:-
- Connection to the Internet: Ubuntu has a very good repository but it needs the Internet to connect to it.
- Update your source list: Ubuntu needs to sync with repository as which package is found where. So from time to time update your source list.
- Ubuntu Software Center: That is the easist way to install sofrware, like someone has already suggested.
I hope this helps