While I was installing Windows XP as a VirtualBox guest operating system I got the following error message from the Windows XP installer "No valid system partitions were found. Setup is unable to continue."

The solution was to start the installer with the /syspart:c: argument.

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Select Disk 0
Clean
Create Partition primary
Active
Assign Letter=C
Detail Disk
exit

Format the newly created c: disk.

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format c: /fs:ntfs /x /q

Start installing Windows

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winnt32.exe /syspart:c: