iuser.dll is a malicious library file, which is mostly installed and used by Error Doctor 2007. This virus is designed to display fake security notifications that all report about hundreds of viruses. After doing that, this rogue is also set to offer its fake licensed version that should never be purchased. If you want to get rid of annoying alerts and similar issues on your computer, you should remove Error Doctor 2007 from it.
As soon as iuser.dll is installed, it is used for implementing a long list of malicious functions. This file can be used for running a destructive payload, acting as a plugin and collecting information about the victim and initiating other tasks. If you don't want to lose your passwords and logins, you should get rid of iuser.dll. For that we recommend using RegCure Pro.
In some cases, a presence of iuser.dll does not mean that your system is infected. This file may actually belong to a fully legitimate application, so you should double check iuser.dll before terminating it.