I have just managed to get rid of Windows 8 and installing Windows 7 in my laptop. I hope this helps:
1.1) Download all required Windows 7 (64 bits) drivers from DELL (http://www.dell.com/support/drivers/us/en/19/Product/inspiron-17r-se-7720). The SATA driver (Serial-ATA_Driver_VWCH1_WN_126.96.36.1990_A02.EXE) is needed while installing Windows. The rest (see the list in Section 3) will be needed after installing Windows.
1.2) Execute “Serial-ATA_Driver_VWCH1_WN_188.8.131.520_A02.EXE” and select the option that allows to extract the contents of the package without installing the drivers.
1.3) Save the resulting files into a flash memory device, within a folder named “Serial-ATA_Driver_VWCH1_WN_184.108.40.2060_A02” (for example).
NOTE: An alternate method (more complex and with few advantages) is making a bootable DVD from the original Windows installation disk for including the SATA drivers. This is explained in “Tutorial: How to clean install Windows on Inspiron 17R SE” (forum.notebookreview.com/dell-inspiron-dell-studio/686824-tutorial-how-clean-install-windows-inspiron-17r-se.html). Instead of using "Ultraiso" (the evaluation license cannot work with Windows DVD because it contains files greater than 300 MB), "Imgburn" (www.imgburn.com/) can be used for making the bootable image. See “How to create a Windows Vista / 7 installation disc (bootable) using ImgBurn” (forum.imgburn.com/index.php?showtopic=11194).
2) Installing Windows
2.1) Plug the flash memory device (including DELL Inspiron SATA drivers) and insert Windows 7 installation DVD.
2.2) Reboot the laptop, press F12 (Boot Options), change the UEFI boot mode to Legacy (Secure boot: OFF) and boot from the DVD. Windows installation process should start.
2.3) After selecting Windows installation language and accepting the license terms, select "Custom (advanced)" installation type. Then the list of available disks and partitions will appear (not all of them will be shown because a proper SATA driver is missing). So:
2.4) Click on "Load Driver", click "Browse" and select "Removable Disk" (the flash memory device). Then select the directory "Serial-ATA_Driver_VWCH1_WN_220.127.116.110_A02\f6flpy-x64\f6flpy-x64". The complete list of partitions should appear after installing the driver.
Notes: If the installation of the SATA driver fails, retry. It is also possible to have unexpected blue screen errors for "iaStor.sys" (address FFFFF880030A043B) when Windows is about to automatically restart, but they can be ignored (seemingly). These problems occurred to me only when using the alternate method (making a bootable DVD including the SATA drivers).
2.5) Select a partition and continue installing Windows normally.
Note: Delete the existing partitions and create a new one if the installation complains about "GPT style" partitions.
3) Installing the rest of drivers
When Windows 7 is completely installed, this should be the situation in the Device Manager:
When preparing (see Section 1), the following drivers for Windows 7 should have been downloaded from www.dell.com/support/drivers/us/en/19/Product/inspiron-17r-se-7720. Installing them in this order worked for me:
3.1) Intel Chipset Device Software (SM Bus Controller): Chipset_Driver_RPHMX_WN_18.104.22.1681_A01.EXE
3.2) Intel Management Engine Components (PCI Simple Communications Controller): Chipset_Driver_MNN8P_WN_22.214.171.1242_A03.EXE
3.3) Intel USB 3.0 eXtensible Host Controller (Universal Serial Bus Controller): Chipset_Driver_M2RXF_WN_126.96.36.199_A03.EXE
3.4) Realtek USB 2.0 Card Reader Controller (USB2.0-CRW): Chipset_Driver_MTT9W_WN_6.1.8400.39030_A01.EXE
3.5) Realtek PCIe FE Family Controller (Ethernet Controller): Network_Driver_P7W53_WN_7.065.1025.2012_A02.EXE
3.6) Intel Centrino Advanced-N 2230 WiFi (Network Controller): Network_Driver_HP8FN_WN_188.8.131.52_A03.EXE
3.7) NVIDIA Graphics Driver (NVIDIA Stereo Controller): Video_Nvidia_A04_W74_Setup_PYG93_ZPE.exe
4) Configuring Intel Smart Response
There is no need to install “Intel Rapid Storage Technology Driver” nor configuring anything. After downloading "SetupRST.exe" from Intel web page, I was surprised to see that the System drive was already being accelerated: