Moderator Post (s) for this thread: >SizebyHKestenholz on 06/13/2009
If I am not mistaken, you will first need to remove the steam vents on each of your radiators so that water will not leak from them and replace them with plugs.
Cleaning the radiators can be as simple as disconnecting them and hosing them to remove rust and debris.
You may have larger problems, however, in trying to re-use existing steam radiators for a hot-water conversion, because since steam operates at a much higher temperature than hot water systems...existing steam radiators may not (and probably will not) supply an adequate amount heat for their size as did the steam.
It may be best to remove the steam radiators altogether and replace them with hydronic baseboard radiators.