You should be able to log into root by default. As the installer of the OS, the root account should be the only one there.
During the install you should have alreasy picked a password.
You will log in as root only.
Once in, you should be able to mount your linux partition and cd/ to it.
Then presumably you need to start making the configuration changes to Lilo, fstab etc.
If you have used distros like Mandrake etc. before, you will have to accept that more work is involved with Slack. But you have better control and stability.
But, as long as you are in as root with read-write proveleges, you should be able to whatever you want now!