Western Europe used to mean "any part of Europe
that the United States of America
can step on as a direct or indirect result of the Marshall Plan
," or "anything west of the Iron Curtain
," or "the European sphere of the North Atlantic Treaty Organization
." But that was then, this is now.
Since the end of the Cold War, Western Europe, following the predictions of Samuel Huntington, has turned into a cultural entity: namely, the Roman Catholic and Protestant half of Europe, as opposed to the Orthodox half. Countries that used to be part of Eastern Europe, such as the Czech Republic, Slovenia, and Croatia, are now being considered by some as part of Western Europe. Serbia and Bulgaria, though, are still firmly in the East and will probably stay there.
Poland is still in limbo: unlike Yugoslavia, it didn't have the luxury of being divided into a Light Side and a Dark Side, and the vestiges of communism still hanging around (not to mention 190-proof vodka) have largely kept them at arm's length from Western Europe. Bosnia is another tough call, because they have several competing cultural identities. Romania is yet another.
It's an Old World, that Europe. The changing political conditions on the Continent have brought an old term back into use: "Central Europe."