To add to miaviator's post - you can choose between Slack and Windows and Slackware comes with the LILO bootloader that will make this possible. No extra software needed for that.
For a first install for a home system you might want to keep it simple and have a single root partition. That can be, say, 2-4 gigs depending on what you install and what you want to add. But you'll probably want to have some space for your personal files. So you could make / bigger or create one more partition for /home and make it as large as you like.
The only gotcha to this is your current filesystem. NTFS is a pain and Slackware can't (AFAIK) resize such a partition. Also, while it can read from it out of the box, it can't (safely) write to it.
So, for this, you'd need ntfsresize (I think that's what it's called) which is open source and freely downloadable, or you could actually borrow the fine services of a Mandrake disk.
Or you could use some proprietary resizer, which I don't recommend. Not for 'religious' reasons but because they often seem to fail with Linux. And, as far as handling the files, it might be best to create a shared directory that's formatted as FAT32 that both Linux and NT can safely read and write to. (NTFS write support is coming along though and you might do a web search on that - there is a project that uses Windows' own drivers to deal with the system, but I don't know much about it. While I have NT systems I don't use them so haven't followed developments closely.)
If you have XP on FAT, none of this applies and it's a breeze.
I have NT5 and 5.1 with Linux on each but all I did was add a second drive to one system and resized while testing Mandrake on the other. Both methods worked great. I've also shared between Win9x and DOS on other boxes and that's even easier.