Why is explorer.exe freezing Windows XP?

When shutting down, my computer is unable to shut down properly. explorer.exe freezes and I am forced to open up Task Manager and kill it manually. Then, the freeze is over, but Windows still won't shut down. Furthermore, once in a while, when opening some Explorer windows, explorer.exe will freeze and I am forced to kill it through Task Manager. explorer.exe shows no CPU activity, no RAM fluctuations, and doesn't read or write any data.