You could try, using the XP CD recovery console and type fixmbr.
this will replace lilo with Windows boot loader, then boot into windows make sure it works ok. If not your installation is corrupt.
once done, insert Slackware CD and run :
login as root and run lilo.
If all else fails, try again from scratch, Ive noticed that some times things just go wrong. But I guarantee it works, mine is setup similar.
As for the Bad block etc, your entry in /etc/fstab should look something similar to:
/dev/hda1 /mnt/XP ntfs auto,user,ro,umask=000 1 0
this is mine set for all users to access the files.