Laptop Problems

I just bought a new HP Pavilion DV6601AX with Windows Vista, but did not get a standard antivirus with it. A few days ago my friend installed an antivirus program on it for me, and it worked fine. However, yesterday, I tried to boot it, and the window screen did show up (as completely BLACK for a while) and then the taskbar came up, but the cursor never stopped being busy. It just showed up as a ‘busy circle’. I gave it some time, but it kept going for half an hour! I really need to work on a word document, but I can’t even click on anything. I restarted it, and the same thing kept happening. I even tried putting the battery in again in case it wasn’t installed right, but nothing changed. What am I supposed to do?
I am also more comfortable with Windows XP and I was planning to get that installed… but I am really scared because I just got a new laptop. HELP ME PLEASE!!! I live all alone in an apartment and I’m not that great with computers.