A trojan (you know, the malicious virus type) for Windows systems.

It has pretty standard features - go through people's hard drives, fill the screen with unclosable message boxes, et cetra.

To remove the server, boot into safe mode (or MS-DOS) and delete the file expl32.exe in your Windows directory.

There is also a registry key that it installs in "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" of your Windows registry tree. It isn't necessary to remove the key to remove the trojan, but it might help your computer run more smoothly.