Is it clean from underneath sis? You can try cleaning it by turnin it over and (if its the ball one) just twist that thing underneath, it will open up and you can clean it with a tissue or smthin..
it could be due to following reasons:
a. Your pc is effected by some spyware(most likely)
b. Your operating system is currupted
c. your mouse is broken down(rarely)
scan you pc for viruses and remove them.
if you r using MS Windows then visiting official website of windows can be helpful.
Most appropriate is formating your hard drive. if you have windows CD then back up all ur data, format ur hard drive and reinstall windows. b4 adding any thing install antivirus. then scan ur backups and after cleaning them for any virus threat put them in appropriate drive. this will refresh ur system n u will observe the enhancement in term of Response.