You may be able to order Windows 7 media directly from Microsoft. This will allow you to install a generic version of Windows 7, without any of the Gateway software or drivers that come included with the computer. This means you'll need to install all the additional drivers and software you'll need from Gateway's website in order to give your computer full functionality.
In your situation, I'd recommend getting a copy of the recovery media for your computer. First, try to boot into Windows and run the Gateway Recovery Management to burn a set of disks. If this fails you'll need to get a set of recovery media either directly from Gateway, or your nearest Future Shop, if they have a set. These disks will automatically install everything -- Windows 7, drivers, and additional software, and make your computer run like brand new.
You can also try using disk cloning utilities, such as Norton Ghost or Acronis Disk Director to try and make a direct copy of your old hard drive onto your new one, but if the old hard drive is failing, then there may be errors during this process, or the whole operation could fail, leaving you needing a Windows 7 disk or recovery media anyway.
This is all assuming your hard drive is actually failing, of course. You may be able to reset the computer's software by running the built in recovery program. When you turn the computer on and see the Gateway logo, hold down the Alt key, and keep tapping F10 until the Gateway logo goes away. If done correctly, the computer should boot into recovery mode, and prompt if you'd like to continue. Note that this erases all data on the hard drive -- make a backup of anything important before doing this.
