Your libary is still on your PC where you put the files. You did not put any files on your MBA except the ones you bought on iTunes. They do not magically transfer themselves between computers. You can do this if you set up Home Sharing.
iTunes: How to share music and video - http://support.apple.com/kb/HT2688 - about Music Sharing and Home Sharing
Home Sharing Support page - http://www.apple.com/support/homesharing/
iTunes: Setting up Home Sharing on your computer - http://support.apple.com/kb/HT4620
Anything (with some limitations) you buy from the iTunes Store is in iCloud.
If you subscribe to Match then things you have are uploaded to iCloud.
CDs you downloaded to your PC are only in the iTunes library there unless you copy the whole library or the media files to the Mac, or you subscribe to Match.
That's helpful thank you. I now get the difference. I have looked on my PC (via Windows Explorer) but finding the itunes library is not simple. There are several files -inlcuding .exe ones. Is there any more adviceyou can give me about which files I need to copy to my Mac?
I will also have a good look at Match to see what that;s all about.
The .exe files are the application files. your Mac will have hte mac version.
Look at this article.
Where are my iTunes files located? - http://support.apple.com/kb/ht1391
Copy the whole folder, complete, to the equivalent location on the Mac (as also indicated in the article). iTunes will then start up with your old library.
What are the iTunes library files? - http://support.apple.com/kb/HT1660
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html