Where did you see the warning?
Clean Installing Windows 7
Warning: all data on hard disk will be destroyed. Backup if necessary
Change boot order in BIOS.
Turn on the computer. When the BIOS screen appears, press Del ,
Esc , F2 , F10 , or F9 (depending on your computer’s motherboard) to enter the system BIOS. The key to enter the BIOS is usually shown on the screen.
Navigate to the BIOS's boot options menu.
Select the CD/DVD drive or USB drive as the first boot device.
Insert the Windows 7 disc into your disc drive or boot able flash into a USB port.
Save the changes of the settings and restart (option in the last tab in BIOS).
Start your computer from the disc. After you have placed the disc into the disc drive, start your computer. When the computer starts, press any key when asked press any key to boot from CD or DVD
- After you choose to start from the disc, Windows Setup will begin loading.
Choose Windows Setup options like language and locale. Once Windows Setup loads, you'll be presented with a window. Select your preferred language, keyboard type, and time/currency format, then click Next.
Click the Install Now button.
Accept the License Terms shown. Read over the Microsoft Software License Terms(EULA) and check I accept the license terms, and click Next.
Select the Custom installation.
Decide on which hard drive and partition you want to install Windows 7 on.
If the hard drive has data on it, delete the data off of it, or format it.
Select the hard drive partition from the list.
Click Drive options (advanced).
Click Delete partition from Drive options. And repeat this for all your partitions.
Now your computer doesn't have any partitions yet, create as much as necessary by following below step:
Select the hard drive from the list of hard drives.
Click Drive options (advanced).
Select New from Drive options. Select the size, and click OK.
Once you've decided on where to install Windows, select it and click Next. Windows will begin installing.
Rest steps are pretty direct forward. Install your device drivers and other software after setting up everything.
Install security software. Windows firewall is good enough to protect you. But do not install multiple AV with real time protection enabled Free alternatives to AVG are Avast or BitDefender .
I suggest to keep MBAM. MBAM's realtime components are enabled only if you activate the premium version.