Enigma

I bought a hp desktop that is not even a year old. It came with Window vista and I've heard great things about it at first. In the first two weeks, everything was find until it started to slow down so I bought norton antivirus to protect my computer. It seems to make no difference. Now it takes more than three or four days for it to finally finish stop scanning. (sometimes, I just give up) Now, for the past few months, my computer seem to have gotten worse. You know how there is a start button and volumn and where our windows hide when we minimize it? Well, mine sometimes dissapear! =O Part of it at least, and its difficult to do anything on it because its more slower and I can't right click to even refresh. I usually have to log off or shut down to have it go back to normal. Please someone! If anyone knows whats wrong with window vista, help me! I'm going to college soon and want to pick the right laptop and program.