Forget about using MiniTool, Easy2Boot, etc. They're great tools, but unless you're a special case situation, you don't really need them. They just needlessly make your situation more complex. All you *REALLY* need is the Windows DVD/USB and a correctly configured BIOS. It's best to just use a burned disc to avoid the "necessary drivers are missing" crap. It is possible that you have a BIOS which doesn't allow Secure Boot (and therefore UEFI, of which SB is a subset feature) to be disabled. Does ASUS have any BIOS updates for your hardware? Have you tried calling ASUS to ask specifically how to set your BIOS up for MBR boot? Did you read your manual thoroughly?
I'm assuming your disc is a standard, uncustomized setup disc, with the exception of the integrated updates, which should pose no issues where installation is concerned.
After installation is finished, you can verify in Disk Management whether you have an EFI System Partition, or a System Reserved partition. If the former, your disk is UEFI/GPT, if the latter it is MBR/legacy.
The partition that Windows is installed to (what will become your C drive) can also act as System Reserved (but without the label), by being marked as both System and Active/Boot in Disk Management. This is what I recommend, since MBR is limited to either 4 primaries or 3 primaries and 1 extended partition, by using your C drive as both System and Boot/Active you will only consume 1 primary instead of 2. However, this isn't the default setup that an MBR 7 will set up on an empty, unpartitioned disk, it will normally create System Reserved for boot files and another partition for C drive (7).
To make this work just open command prompt while booted into the USB (make sure you boot the USB in legacy mode) by pressing Shift + F10. Then:
2. list disk
3. select disk (insert your HDD's # here without parentheses)
4. clean (this will erase all data/partitions on disk so remove anything you want to keep)
5. create partition primary
6. select partition 1
7. format fs=ntfs quick
8. assign letter=c
9. active (this marks the partition as bootable, forcing Windows to install the boot files into C drive instead, and making C act as both the main 7 and System Reserved partition)
Then just use the GUI installer and install as usual, selecting your created partition. This setup will work fine if you really just need 7 installed and only uses 1 partition. You can always shrink the partition size later if you need to create more. This is about as simple as it gets.
Hope this helped!