I am having the same problem & Ive v_i_r_u_s scanned alot & found nothing.
I know one thing thats making my PC freeze, and its not enough disk space (which is my main hassle with computing.. Oh come on.. who gets a computer with a TOTAL amount of 7GB disk space.. My eyepod holds more than that!!.)
But do you have the latest version of Java?
How much RAM do you have?
Do you have a low amount of disk space? (this depends on what you consider low and what your total space is.)

Also heres another thing to consider.. Does your PC freeze up mostly when you try to launch a certain program? (Mine does when I launch LW)