Here we go..
I had a dual boot system with ubuntu and windows xp, using GRUB. Yesterday i decided i wanted to try slack because a friend of mine had told me a lot about it and how good it was. So i deleted my ubuntu partition and installed slack on that..the install went okay and it told me to reboot, when i rebooted i got the error GRUB error 15, and then the second time i got GRUB error 17. my computer stalls and does nothing after that. I used knoppix last night and saved all my data that was crucial from my XP partition. any suggestions of how to get this working?

Don't worry about your data, it's just the bootloader not working because it needs to read a file from the disk (menu.lst) that's not there anymore (it was when you had ubuntu :P).
You can recreate a working GRUB config file or use LILO instead, which is Slackware's default bootloader.
To install it, boot with Slackware CD 1 and on lilo prompt (when it says boot enter: bare.i root=/dev/hdXX
Replace XX with the correct letter and number of your Slackware root partition, press ENTER and you'll be running Slackware. Once you're there, you've to install LILO, to do that, run: liloconfig

Edit: Forgot to say, don't worry about the warning, choose Expert mode and then install to MBR.

I believe you installed Slackware and during the install, either selected a "safe" lilo installation, or told the installer to install lilo into the root partition instead of the MBR (master boot record).
Grub which you used before as the bootloader, is partly present in the MBR, but the rest of Grub was available in the /boot/grub directory of your Ubuntu partition, and you wiped that away.

What you can do is boot again from the Slackware install CD1 and when it pauses to let you select a kernel, pay close attention to what is on the screen. It will tell you that you can "boot your current installation in a pinch" or words like those, and tell you to enter
to boot directly into the Slackware that is installed on hard disk. Probably the partition your Slackware is installed is hda2, in that case the command line to enter would become
- you'll get the idea.

Once you've booted into your installed Slackware and logged in as root, open the file /etc/lilo.conf in an editor (vi or pine or whatever) and look for the line
(perhaps that 2 is another partition number in your case). You should install lilo into the MBR which you do by changing the above line to
Save the file, and install the bootloader by running the command This will wipe the remnants of Grub from the master boot record.
Now, reboot again, without the CDROM in the drive, and Slackware's LILO boot prompt should appear.

Eric