The AtheOS 0.3.7 download is pretty slim for a self-hosted, full Posix/GUI magnitude OS, as it is only 22 MB for the base installation, plus about 20 more MB for the (unavoidable) C/C++ developer's tools (gcc, NASM, binutils etc). Installation is not really difficult, but it surely is not for the faint of heart or for those who have never installed anything else than Windows before in their lives. The installation process involves 3 floppy disks, a fat partition (used only to hold the base installation tarball), initializing the partition of your choice as AtheOS-fs (a small, 500 MB, partition should be enough), untaring the base installation package to the new partition and then configuring GRUB to boot load the AtheOS kernel or even install GRUB on the MBR (maybe the configuration of GRUB can be the most tricky part for most users in the whole installation process).
After you do your necessary configs on GRUB, you are up and ready to select AtheOS in your next boot. AtheOS loads in full GUI in less than 8 seconds in a modern PC (around 16 seconds in my old and slow laptop). You will soon be presented with the login screen window (AtheOS is fully multiuser, following the Unix model) and here you go in the AtheOS main desktop.