The Classloader files are Internet Explorer (IE) homepage hijacker trojans that infect IE through malicous web pages, which host exploits targeting the Java classloader byteverify vulnerability in unpatched versions of IE.
These trojans usually change the browser start page and search settings and download other trojan/spyware components on the system.
These trojans are distributed on malicious web pages that contain references to the trojans. Usually the classloader files are in a .zip or .jar archive along with several other files.
The Classloader based trojans install themselves from a malicious web page that contains a reference to the trojan.
The trojan uses a vulnerability in the classloader system of Microsoft Java runtime, that allows the malicious applet to break out of the sandbox, and gain same access as any other executable running with users permissions.
After being executed these trojans usually download executable components that are either further parts of the trojan or spyware being dropped by the trojan.
Using the latest version of Internet Explorer and keeping up-to-date with the latest security patches is the easiest way to protect against possible infection from Classloader trojans, as this fixes the web browser vulnerabilities needed for the trojan to work.