Weather you use a virtual install, or a full install depends how comfortable you are with re-partitioning your hard drive.
If you have never partitioned a hard drive, I would suggest you get your feet wet on a virtual install first. I use Virtual Box to run windbloze, and it is not difficult to install an operating system, if you follow the install instructions. I have never used VMWare, so no comment there. If you go this route, the Slackware install docs do not cover a virtual install. Basically, you would do all the steps, except disk partitioning and formatting; and you would not have to install a boot loader, the default being Lilo on Slackware.
If you decide to re-partition your hard drive, you will need something to shrink your windows install. It probably owns the entire hard drive now. There are some nice live distros you burn to a CD, and then run graphical tools to do the shrinking, partitioning, and formatting. G-parted and Parted Magic are two that come to mind. You need about 10 gig for a full install. I would suggest more, say 10 for the root, a small swap partition, and as much space as you can afford for your /home partition.