a) A good idea but they will not slow a computer up, it is usually applications that do that.
b) stick in as much RAM as it can take. RAM is cheap at the moment and 2Bgs would be good..
c) no need. You can scan for spyware/viruses etc but I would save all the documents etc. and reformat the computer. This will give you a brand new computer and with the extra RAM, it should fly.