So you had a windows computer before?
See here for moving content options
When the content is moved over if it has been purchased with the old ID than it will not play until you authorise the new machine with the old Apple ID
Store > Authorise Computer.
This will not change buying content with the new iD going forwar it will just let you platy the old content as well as any new you might get.
If you are going to get rid of the old computer then it is a good idea to
Store > Deauthorise this computer