Okay boys 'n girls, I made installation successfully but not from linux but from windows virtualbox with 'WinToFlash' program.
-- link in case somebody needs it.
Nice automatic program, select drive 'n path to extracted .iso and just run it. Boot, select mode 1 and after installation succeed and auto-reboots, boot from drive again and select mode 2 -- if giving error then mode3 will do it -- and continue installation.
So thats for installation this piece of ****, booting is another story and solving at this moment. Took few hours to boot my slackware again since winsh... rewrited MBR (knew it will happen but since I'm newb it took some time to fix that... thanks to luck I have my Debian live USB... have saved me so, sooo, sooo many times... made it for fun few months ago! Hoooray to Debian live USB!)
But in case somebody have issue with MBR, I booted to Debian, mounted Slackware partitions and chrooted in 'em and ran command 'lilo' -- which owned MBR back to him from windows.
mount main system partition(s) somewhere.
mount /dev/sda1 /asd
chroot /asd /bin/bash