cannot install with an already installed GRUB, Lilo, etc

My system was installed with Ubuntu Linux 6.x and GRUB as boot loader.
I deleted and recreated a partition in Reactos Installer.
The setup seemd Ok but GRUB was still in place with an error 15.
Tried to reinstall a Windows 98 MBR.
Then Boot was also impossible:
message "Node is not a directory"
cannot find freeldr.ini file.

My system is a compaq Deskpro EN with PIII 650 MHz 192Mb RAM 6Gb Disk