Use your Win 7 DVD to "clean" your hard drive.
GO TO A COMMAND PROMPT
USING YOUR WIN 7 DVD/UFD
BOOT TO A COMMAND PROMPT USING A WIN 7 DVD/USB STICK
Ø Insert your Win 7 DVD or Win 7 USB
Ø Power down your computer.
Ø Power up your computer.
Ø Boot up your computer from the Win 7 DVD/USB stick.
Ø When a screen is displayed asking for the Language to Install,
press the key combo of SHIFT + F10.
Do NOT click on Next.
Press the SHIFT + F10 key combo.
You will have a X:\Sources> command prompt.
X: is a ram drive created by Windows 7.
X: contains a mini-version of Windows 7 called a PE (pre-execution environment).
If you do not plan on installing windows at this time, you may now remove the DVD/USB stick.
If you plan on continuing with an install, then do not remove.
Execute the desired commands.
Use the EXIT command to return to the Win 7 installation procedure.
NOTE: UFD = Usb Flash Device = usb stick = pendrive = thumb drive
That showed you how to get to a command prompt.
Here is how to prepare the hard disk once you are at the command prompt:
type these commands,following each by hitting the ENTER key:
SELECT DISK 0
DETAIL DISK (to make sure you got the hard disk)
EXIT (this one gets you back into the install program.