Will not reboot

Running XP home SP2. Have recently had a problems restarting after
installing an update - totally automatic so do not know any details of
update. When the computer reboots to finish the update it hangs whilstg
loading. The only way to solve the problem is to press reset and start
windows with an earlier configuration. Then Windows tries to install the
update again and I am going round in circles. Please help I am getting very
frustrated.