I'm going to recommend you go here -->http://slackbook.org/html/book.html#BOOTING
Read the entire section on booting before you start to modify your system. Section 7.3 deals with dual booting.
There are many ways to make dual booting work. Installing Lilo as the system boot manager is the recommended way, for Slackware, if windows was installed first.
There are other boot loaders, like Gurb and Grub2. You may even get others telling you to install one of them. It is a matter of choice.
There may be terminology you don't understand. Ask, before you try to install Lilo, or what ever boot loader you decide on.
If you install Lilo to the MBR ( Master Boot Record ), then you have to set it up to load windoze. If you do not, windoze will not boot. The rescue disk for windoze will over write the MBR with a windoze boot loader. Then linux won't boot.
So, understand first, then do your install. Feel free to ask, even if it seems like a simple question.