Eclipse always runs all projects - how can I only run one?

OK, I'm an Eclipse noob, just working through the first couple of Plug-In creation tutorials and I have this really annoying problem: I can't figure out how to stop Eclipse from always building all projects when I only want to run one of them. Let me elaborate: I have project A (Java), B (Java) and C (Python). They're completely unrelated, i.e. there are no dependencies whatsoever between them. Project A even lives in a seperate working set. Now I try to run project A, but I get error messages about problems in projects B and C - why is that? How can I only build the current project? There's also another problem that's probably related: When I start my plug-in as an Eclipse application, all the other plug-ins I wrote before are also included in the Eclipse instance that is started. Is this a seperate phenomenon or does it follow from my first problem?