Wandering mouse

Hi there, my mouse pointer keeps wandering on the screen. At times it will be
still, and then when in use, it will just start wandering. Each time I click
the mouse pointer has wandered somewhere else.
I have bought a new mouse hoping that this would solve the problem, but to
no avail.I now suspect that this is a software problem (possible virus?) or
corrupt mouse controller? The mouse is a PS2 optical wired mouse. The
operating system is Windows XP service pack 3. What must I do? Please help !!
Greetings.
Frank