Given the age of the system I would replace the motherboard battery.  If this battery is failing it can cause a variety of boot up problems including the system freezing like you are experiencing.  A new 3-Volt, CR2032, coin cell battery can be purchased for around $3 (US) and is available at any Radio Shack, computer stores and most discount stores.  You can click here for additional information from your model's on-line Service Manual on how to change your battery.  The changing of the battery will not effect any of the data stored on your hard drive if that is a concern.