Mouse is fine but software seems to be messy

Hi

just yesterday I started having problems with my laser mouse. I run XP pro on a home machine. My left mouse button stopped working, intermitantly coming back on again. I swapped the primary and secondary functions over in mouse properties but this is only a temporary measure, as it still calls up menus like secondary function button would. To top this off when I am enjoying a short space of it working normally it gets stuck, selecting loads of icons or dragging them about. I thought it was a virus but have scanned the computer with various software and nothing has helped. Any ideas?