this pc tower no restre disk ,how t back up ystem

going to purchase this small tower
HP Refurbished Compaq DC5800 Desktop PC with Intel Pentium 2200 Dual-Core Processor, 2GB Memory, 80GB Hard Drive and Windows 7 Home Premium (Monitor Not Included)

Support and Warranty:
•90-day limited hardware warranty; 24/7 technical assistance available online or toll-free by phone
•Restore discs are not included. We recommend you use the installed software to create your own restore and backup DVD the first week you use the HP Refurbished Desktop PC with Intel Pentium 2200 Dual-Core Processor, 2GB Memory and 80GB Hard Drive.

How can I back this up through a free back up program , or a flash drive? 8GB

It says it comes with software so I suggest you follow the prompts. I personally don't recommend backing up your system to a flash drive as they are not intended for long-term storage. They are not very robust - they can easily be lost or damaged - or sent through the washing machine.

Instead, I recommend backing up to second hard drive, and/or burning an image to a DVD. W7 comes with backup software or it looks like HP will provide it - perhaps via a recovery partition. Or there are several good backup program (free or paid) available. Many like Acronis.