Nslookup.exe is legitimate system file from Microsoft Corporation that can be found in this subcategory: %SystemRoot%\System32. It is a digitally signed file, so we don't recommend removing it from the system. According to the publisher, it allows users to know what IP address a domain name resolves to or what domain name an IP address resolves to. If you stop Nslookup.exe, you won't have a possibility to use the nslookup tool properly.
Virus creators may try to use the name of this file to disguise their malware. Typically, malicious files are not stored in %SystemRoot%\System32 subcategory, so if you find Nslookup.exe in other location, you should scan your computer with reputable security software. At the moment of writing, we don't have information about viruses hiding themselves under the name of this process.