First, the computer should not "freeze". If it does you have serious problems - either hardware or software which you need to correct.   A Windows XP system should never freeze.

However, if a PC 'freezes" you can push in on the power button and after about 10 seconds it will power off. 

And, you are correct this is not the right forum section.  This is the Modem section and this has nothing to do with the modem.  The General Hardware section would have been the most appropriate section for your question.  I see this is your first post - always try to post in the most appropriate section, not just whatever section comes up - it does make a difference to get assistance.