I am a non-Muslim . If my muslims friends greets "Assalamualaikum" , would it be haram if i reply with "Waalaikumsalam" ?
If you know you should believe in Islam but you don't find enough courage in yourself to confess in Islam, and want to behave like a Muslim instead, not to commit anything wrong at least, things would be different than when you are merely interested in Islam and like to interact with your friends in their own manner like you do the with your friends of other nations/religions.
If your case is the former, understanding the truth and denying it, or having reluctance to embrace because of fear of its side effects on your current life style and communications with family and friends, or just simply postponing the issue of making decision about it to tomorrow (a tomorrow that may never turn into Today), that would be considered as wrong (like Haram that you said) even from non-Muslims.
If your case is the latter, that would be nice of you to respect their way of living and it will not be Haram or even wrong. That shows you are not an arrogant person and Allah likes it for you and all of us. In a Hadeeth from the Holy Prophet peace be upon him and his household we are told that after believing in Allah nothing is more be liked to Allah than us being tolerating and affable with respect to the people He has created, not even only with respect to the other believers but to all.
"Assalamualaikum" means "Peace be upon you" and "Waalaikumsalam" is a reply "Peace be upon you, too". These are normal arabic words. If you like it, you can say it.
This is very old tradition of different prophets. Quran mentions about the greeting by Prophet Abraham (PBUH).
And certainly did Our messengers come to Abraham with good tidings; they said, "Peace." He said, "Peace," and did not delay in bringing [them] a roasted calf.
All the major religions have similar words for greetings. Prophet Muhammad (PBUH) sanctioned this tradition and now it is global greeting message among the muslims all over the world.
No, in fact your friend might even be pleased that you made an effort to learn more about his/her faith. Comment by MAKZ stands true also.
In case you are curious,
AsSalamu Alaikum means
peace be with you and the appropriate response to/for a muslim is
wa Alaikum Assalam ie
And onto you peace. This is the greeting that prophets like Jesus ,peace be upon him, also used in his time.
Peace be with you!