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.

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

format c: /fs:ntfs /x /q

winnt32.exe /syspart:c: