Boot sector question

I have a floppy disk that boots MS-DOS and executes some commands.

The last command I want it to run is something that loads the hard disk boot sector in and jumps to it, thus relinquishing control from DOS.

My code is:

mov ax, 201h
mov bx, 7c00h
xor cx, cx
mov es, cx
inc cx
mov dx, 80h
int 13h
jmp 0:7c00h
but this just hangs. Any ideas what could be going wrong?

>>2
Thanks for your suggestions. However I've narrowed the bug down to when the Windows-installed boot sector copies itself from 0x7C00 to 0x600. It appears to overwrite some DOS structures which causes the system to hang. I expect DOS has hooked some of the BIOS interrupts so the boot sector code ends up calling it instead of the BIOS. Not sure yet though.