JS_Dloader.NSP is malicious JavaScript that may be downloaded from a certain URL.

It exploits a vulnerability in the instant messaging application Yahoo! Messenger. When successfully exploited, it allows this malicious JavaScript to download a file that is detected by Trend Micro as JS_AGENT.TEJ. As a result, routines of the downloaded file may be exhibited on the affected system.

Technical details can be found at this Trend Micro page.