How can I completely format my C drive?
Usually when I install Windows I let it do the format, but it seems to keep a Windows.old folder and other folders that were already on the C drive. It doesn't delete those. How can I do it where it deletes everything on the C drive?
When Windows 7 setup loads and you are at the screen where it shows your hard drive partitions, click the "Drive options (advanced)" icon. Now you can delete and format your partition(s) of choice. Be sure to pick the correct partition if you have more than one. If there is more than one and you know what they are and feel safe that they can be deleted, delete all partitions on Disk 0.
Once you click that, you will get this screen where you can highlight partitions and delete them. Once deleted they will show up as unallocated partition. Then format if you wish, but format is not necessary on unallocated partitions, and I never do it. I just highlight the unallocated partition and hit the Next button, Windows 7 will take care of the rest. Your partitions before deletion may look different than the screenshots I posted here.
