These are the minimum system requirements for Ubuntu Linux Desktop and Server Edition as of June 5, 2011, Make sure your computer’s CPU, RAM/Memory, Hard drive, and videocard are fast enough an meets the requirement for Ubuntu before you start the installation of Ubuntu on your home PC
Recommended Requirements for Ubuntu Desktop edition
Processor/CPU 1GHz(x86)
Memory/RAM 384MB
Hard drive/Storage 5 GB
Video Card Capable of VGA 1024×768
Minimum Requirements for Ubuntu Server edition
Processor/CPU 300 MHz (x86)
Memory/RAM 128MB
Hard drive/Storage 1GB
Video Card Capable of VGA 640×480
http://en.wikipedia.org/wiki/Ubuntu_%28operating_system%29#System_requirements
A CD drive or USB flash drive is also needed to start your computer to begin the installation task from the Ubuntu install CD or bootable USB flash drive.
I also recommend your PC has a network card to plug into your router, or high speed modem to perform an update of Ubuntu after the installation.