I've had this same problem during the installation attempt on my Windows XP (service pack 3) system.
I have a Sansa c250 with the latest (Sansa) firmware.
Using the Rockbox utility, the autoinstall will install Rockbox but not the bootloader ("Detecting Sansa--No Sansa").
So how does one go about temporarily disabling the USBDLM (I don't see it active via the Task Manager, fwiw)? I've tried the manual install but that won't work either.