Fix “Show hidden files and folders” option of Windows Explorer View settings
Some viruses disable the option "Show hidden files and folders" of Windows Explorer (available from "Tools/Folder Options…" menu, at "View" tab) so that when user changes that setting it is ignored and keeps its previous value.
A quick fix is to use the registry editor (go to Start/Run… dialog or Vista/Win7 Start menu Search box, type RegEdit and press the Enter key), use the treeview at the left side of the Registry Editor window to navigate to the key:
Then right click the value "CheckedValue" at the right side of the Registry Editor window and enter the value 1 or 2 to show or hide hidden files and folders respectively. Viruses seem to place value 0 there to disable that option (not sure if this is a Windows feature, looks more like a bug of the respective dialog to me).