In this situation I normally install Windows first (try to minimize the size...depending on your needs). I'll then install slackware. When using fdisk, I normally create an extended partition using the remaining space on the hard disk, and then create logical partitions within the extended. Make the main logical partition (the one you will label as 'root' during the installation) bootable. Then, when you insall LILO (unfortunately, I believe it to be the only solution unless you will use another boot loader -- the Windows Me bootloader, to my knowledge, can't boot multiple OSes) the Slackware installer will see the "DOS" partition and ask you if you wish lilo to add this into its booting scheme. Select yes. Be sure to check www.tldp.org
for a how-to or mini-how-to. There is lots of documentation on the web regarding this subject. It will take a bit of research, but any further info should be easily available.
Here's an example: