I could not format C while in W7. The other partitions, were allowed.
I used XP as a tool to zap the W7 install, then did a full Format. Reinstalled W7 fine.
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Is your OS on disks? Indeed, two disks? On my current PC - which is an HP, I have a W7 that was produced as an
upgrade for HP Laptops. The 'Installation Disk' refused to let me proceed, saying it was not a laptop. With a clean format, I used the second disk and completed a new install.