I don't want dumb stuff like this to happen, causing MSN Messenger to take 1.5 GB:
How can I make sure that only my browser is able to run Flash?
I dont know if there is a way to actually do this, because MSN might come with an own flash installation or may prompt to download flash if it is not installed.
What you could do however, in case you are using
If you do not need it anywhere else, uninstall the ActiveX version of Flash Player. Modern browsers (except IE 9) use the NPAPI version, which can be installed independently.