Downloader.NLK is a hacking tool. These are programs designed to enable malicious users to cause or facilitate action detrimental to the target computer.
However, under certain circumstances (for example with network administrators), they could be used to check the security status, possible attack channels (in order to minimize risks), etc.
These tools, even though they are not by nature viruses, are considered as dangerous to victims of attacks.
Downloader.NLK does not spread automatically using its own means. It needs an attacking user's intervention in order to reach the affected computer. The means of transmission used include, among others, floppy disks, CD-ROMs, email messages with attached files, Internet downloads, FTP, IRC channels, peer-to-peer (P2P) file sharing networks, etc.