Spyware and adware slow your computer because they are running and are monitoring infomation you have so compineys can sell you stuff better. And yes it is legal for compnies to do that.
ps. Dont get huntbar....thats a baaaaaaaaad spyware.
To get rid of it you might have to go into regedit and do some vary carful deleating, but dont do that unless you know whut to deleat, you deleat the wrong thing and your computer is dead!