Troj/NtRootK-F is a backdoor Trojan for the Windows operating system.
The Trojan hides processes by directly manipulating internal kernel structures.
W32/Sdbot-PK is a member of the W32/Sdbot family of internet worms that spread by scanning for and exploiting known vulnerabilities and weakly protected accounts.
The worm connects to a remote IRC server and enables a malicious user to remotely control an infected machine.
W32/Sdbot-PK drops Troj/NtRootK-F as the file msdirectx.sys which it employs to hide its process.