Did you resize your HDD?
I would suggest that you use the Disktools available to you within Vista to resize the HDD. First defrag the Vista NTFS drive.
You can get to the disk management by;
Start > Right Click Computer > Manage > Disk Management
Be very careful when you use the tools. You can now select your disk and resize to allow space for a new install of your distribution. You will need space for your swap and the Linux.
With 2GB ram you probably won't swap but I would make a swap space. I would make sure to allocate enough space for the install and growth.
You should now be able to boot the Slackware cd1 and perform the install.
Did you read the Slackware 12.0 CHANGES_AND_HINTS.TXT
These links and others can be found at 'Slackware-Links