Is it possible to completely uninstall windows?

There isn't an uninstaller, unless you consider format c: to be uninstalling :)

Format, install alternative OS (can probably format from the boot CD/DVD of alternative OS)
 
Most OSs have their own filesystem anyway, so formatting is good.
If you are looking at keeping all your files, then just partition on the fly, copy them over, then format the system partition.
 
Whatever the end objective is, dban would indeed remove windows.

It seems likely that the OP wants to reformat and then reinstall windows, in which case just stick the disk in the drive and choose to reformat. Back your data up first.
 
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