I think that NTLoader is going to be your best option
Most likely.
If you follow the dual boot FAQ for NT and Linux you should be able to boot both OSes without entering BIOS.
http://www.tldp.org/HOWTO/Linux+NT-Loader.html
In general, you will need to grab the bootsector from the slack distro and make it available (copy it) to your windows c:\
Not knowing your exact disk setup, it's hard to give exact directions on the process.
The advantage of this method is that you will not have to worry about copying over the MBR if you ever have the need to reinstall windows.
General directions follow:
Boot into Slack
#dd if=/dev/hdx# of=/root/bootsect.lnx bs=1 count=512
where the x and # in /dev/hdx# are the correct drive and partition where lilo is installed.
That should create the file named bootsect.lnx with a size of 512b in the /root directory.
Find it and copy it to removeable media that windows and linux both can read. ( I use USB Memory stick, but have used floppys, and CD-RW also)
Once copied, boot into windows, and copy bootsect.lnx to c:\
Then you must have rw access to boot.ini (a hidden system file located at c:\).
It looks something like this:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Home Edition" /noexecute=optin /fastdetect
c:\bootsect.lnx = "Slackware Linux"
just add the last line and set your system to boot from your windows RAID.
On reboot, it should give you the option to boot both windows and slackware.