The Linux filesystems are in VirtualBox ".vdi" files on the Windows XP system, which I was able to locate after booting with the Slackware CD. I've been looking around extensively for info about how to set about this. This does not seem to be straightfoward unless the VDI file is transerred (somehpow ... ) to another machine on which VirtualBox is installed.
I think that's the only practical way, copy the .vdi files to a system which has VirtualBox installed and then get that system to boot the virtual linux.