Unable to format C drive

Actually previously I was having Win7 Starter on my laptop which was installed on C drive, but somehow it got infected by virus which I was unable to remove. So I tried to install new OS on C drive but I was unable to do so. Then I tried to install new OS on the drive other than C and installed. But now I'm unable to format the C drive in which previous OS was there. Whenever I try to format C drive it is showing message window like this, "Windows cannot format this drive. Quit any disk utilities or other programs that are using this drive, and make sure that no window is displaying the contents of the drive. Then try formatting again". I think old OS is still there. How can I remove the old OS from C drive? Please suggest!

Thank u Mr. Dwarf.
The newly installed OS is on "A" drive.

You should reinstall, delete all partitions then click next. Never name your partition A:

You are booting from the dvd yes ?

Gregrocker has a link for a perfect install.

On many new HP's the Recovery partition will still run from it's hotkey even after a clean reinstall to C. You can try that now by tapping the F11 key at boot, or booting the Recovery Disks if you made them first.

If HP Recovery will not run then you don't need any partitions on the HD now. I would wipe the HD first to get the cleanest start, using Diskpart Clean Command from the DVD accessing DISKPART At PC Startup.

Then follow these steps to get a perfect Clean Reinstall - Factory OEM Windows 7.