I have unzipped the file w/ winrar. I have downloaded it as .rar file. You cannot unzip the .demo file. I also posted in a different forum and got the answer that I have to install doom 3 or at least the alpha to use this file.
AMD Athlon XP 2700+
Thermaltake Volcano 7+
2x256 MB PC2700
Creative GeForce 4 Ti 4800 (4600 Chip)
WD 80GB/8MB HDD