Infected.

Hey guys,

 Can someone please help me, I think my computer is infected. Sometimes when Im typing the computer would freeze for a while. Moments after I will see all letters that I had typed while it was frozen coming up simultaneously. Recently, my cursor would also freeze at times and I will have to restart the computer. I recently bought the laptop and its loading a lot slower than  when I first got  it.I don't keep a lot of file nor download much information onto it (all is school related). However, when I run the Norton Internet  Security 2012  all it ever finds is cookies but no virus. Can someone please help me clean my laptop.

 Thanks