Download an official Microsoft .iso of Windows 7 and burn your own installation DVD click here scroll down to the English(USA) Media Refresh (SP1 U) versions as they already have sp1 integrated with W7. Use your own product key that should be on the coa sticker on base of laptop.
You will need to find and download all the Drivers ready to install them as well for your laptop's motherboard chipset and hardware.
May I ask what state your old hard drive is in? is it bootable? it may be possible to restore it back to factory fresh if you tell us what make and model of laptop it is.
If you are wanting to replace it with a bigger harddrive then it also may be possible to clone your existing old one. This would be easiest way.