If the radiators are leaking from the cores, between the tubes, its very difficult to fix them. You can buy replacement radiators at home salvage yards. If the raidators are leaking at the joints between the pipes and the radiators its a little easier fix. You have to shut down the heating system, (not something for winter) then drain the system down, clean the buildup off the ends, dope them and put them back together. You will need a large pipe wrench and or radiator wrench to work on the pipes. A simpler solution, and one guaranteed to work, is to call an experienced plumber.