My hard drive is 160 gb but it just show to have 5gb

How much do you have installed?

This is freeware and will show you how much each folder has
JAM Software - Windows Freeware (http://www.jam-software.com/freeware/index.shtml)

This way you can see where all the gb's are being stored at, if not shown when running this where you may have 155 gb, you may have a lot of restore points that you can remove that can get a lot of that back if that is where your gb's are being used

What os are you running so can tell you how to remove old restore points

But check out software above so you can maybe narrow down where the space is being used

If it's a new install you might want to check the manufacturers website for known issues with capacity. Seagate has had several problems with firmware causing only a limited amount of capacity the be seen when running in a Windows environment.