If your old PC is a store bought OEM PC that came preinstalled with Windows 7, then yes you'll need to purchase a new copy of Windows 7 to install and activate on your new PC.
If your old PC used a retail copy of Windows 7, then you could use the same installation DVD to install and activate Windows 7 on the new computer. You will of course need to remove Windows from the old PC.
thankyou Shawn,,, yes i bought the original disc,,,, but how do i remove it from the old pc, it is toally kaput, it wont even start up,,,,, i was going to put a hammer through it and put it in the bin,,,,?
You might wait before smashing the old PC's hard drive to see if you may be able to connect it in the new PC to see if you may be able copy anything you want to keep from it to the new PC's hard drive. Heck, if the HDD still works, you could format it and use it as a data drive in the new PC.
Are you wanting to move a retail copy of Windows 7 from the old PC to the new one? Be sure to delete it's partition on the old PC once you know it will install to the new one.
What OS originally came on the old PC? You can reinstall that if you want something on there and to take advantage of the license which came with it. A Clean Reinstall might even rejuvenate it. Follow these same steps for Clean Reinstall - Factory OEM Windows 7