Problem with Logitech PS/2 Optical Mouse. Violent Scrolling without control

I have a logitech optical ps/2 mouse. I am struggling with a strange problem . . . the mouse scrolls automatically & very violently from top to bottom and also horizontally in all applications. I was using the mouse without any problem for last 8 months .. without any paper or mouse pad below it. I also tried re-installing the Win Xp Pro OS suspecting it as virus attack… but same problem persists. Norton antivirus 2005 could detect any virus either. Can any body help me with this?? Do i need to replace it???