The 25Gb partition is an Asus Recovery partition, which contains an image of Windows modified by ASUS. But as you said, the F9 trick is not working so we'll get rid of it. After installing Windows 7 we can create a recovery partition if you want.
If UEFI and PXE are both disabled, then we are free to continue.
- Now let's make a low-level format of your Hard disk.
- Please boot again from the newly created USB/DVD to install WIndows 7. Confirm the language / regional options and press Next. The menu with the install button will show up, do not click the install button yet.
- Press the Shift + F10 button combination. The Command Prompt window should show up. Inside it, type "diskpart" (without the quotes) and press Enter.
- Type "list disk" (without the quotes) and press Enter. A list with disks would appear. Check their sizes to see on which disk you would like to install Windows 7. I would recommend you to repeat the same procedure below for every disk that is inside the PC. Notice it's corresponding number and if it shows as online or offline.
- Type "select disk #" (without the quotes) (where # should be replaced with the corresponding number of the disk). If the disk shows as offline in the list, type "online disk" (without the quotes).
- Type "clean all" (without the quotes) and press Enter. This will take about 5-6 hours for big disks, you'll have to wait. Do not stop the low level format while it is running.
- Repeat the procedures for all internal harddisks, then type "exit" (without the quotes) twice. The command prompt should close. Close the installer program and turn off your PC.
- After doing this for all internal disks proceed with installing Windows. Boot again from the DVD/USB, select your language / regional settings, click Next, and then click the install button. Select Custom install. Select the unallocated space, then click the Next button.