In Partition Magic there is a utility called pqboot.exe. You run it from a floppy diskette or as I did you make a 40meg primary dos partition and run it from there. It allows you to set one of 4 primary partitions on a disk to be c:
NT based OS's are not completely fooled since the boot.ini give the correct partition number.
It makes no difference to the OS if its on c: or z:.
The system drive is the drive containing the boot files ntldr, ntdetect, boot.ini or msdos.sys/io.sys and command.com.
When you multiboot the system drive IS C: !!!
Don't confuse that with the BOOT DRIVE which in the case of XP is d:
Don't forget now, in MSspeak, system is where the boot files are and boot is where windows/winnt folder resides.
Might check out online the many articles on multibooting. That was old school. New school is virtualization with the freebies from MS and Vmware. You load the other OS's on a host OS like XP.
Are you ready for where Microsoft wants you to go today?