If you haven't installed many applications, in particular Desktop apps, I would recommend re-installing Windows 8 into the larger partition.
Start Windows 8 installation by booting from DVD or USB.
You can edit the disk right from the setup wizard. When it asks you where to install Windows, you can remove both
Windows 8 partitions and create a new one which would span all the free disk space.
Alternatively you can remove the small partition after you install a new copy of Windows 8, and then enlarge the only remaining partition to the end of the disk.
If you want to keep the existing installation of Windows 8, then it's trickier.
The first thing to do is to move the installation of Windows 8 from the small partition to the larger one. You cannot resize a partition moving by moving its beginning. (Disk Management tool built into Windows cannot do this. I'm not sure if other software can do it.)
This task can be accomplished by Acronis True Image or other software.
Then you would need to restore the boot loader. This task can be done using Windows 8 install media.
An easier way would be to create system image with Windows tools and use it to re-install Windows on another partition.
- Create a system recovery drive:
- From the desktop, click Action Center icon on the taskbar.
- Click File History on the left pane at the bottom of the window.
- Click Recovery on the left pane.
- Click Create a recovery drive.
Follow the wizard to create a recovery USB or CD/DVD.
- Create a system image:
- Open Action Center from the desktop.
- Click File History on the left pane.
- Click Windows 7 File Recovery on the left pane.
- Click Create a system image on the left pane at the top of the window.
Follow the wizard to create system image.
You can save it on the other partition of your hard drive where you have Windows 7, or you can use an external drive or a network location.
- If you save the system image on your hard drive, copy it to the current Windows 8 partition. You may need to give access permissions to the folder where Windows stored its image.
- Boot your computer from the recovery drive you created above.
(The original install media you used to install Windows 8 may be used for that, although it may not work.)
- Select recovery options and point to the system image you created.
Then wipe out your Windows 7 installation and reinstall Windows 8 from the system image.
Disclaimer: I cannot provide the detailed instructions for the last step as I haven't done it before myself. Theoretically it's possible. And I restored my Windows 7 installation using the system image stored on an external hard drive, although I initiated it from a working Windows version, and I wanted it to fix the existing installation.