Problem is that printer does not have Win 7 drivers, it supports only up to Vista.
You could try with the Vista drivers (try in compatibility mode) , but there is no warranty it will work.
The other option is to try through Windows add printer Wizard, and search Windows update for drivers.
Before trying any of these options uninstall previous drivers.
Here is the link for driver Downloads:http://support.lexmark.com/index?productCode=LEXMARK_X1185&linkSelected=node2
Source: I work as Lexmark technical support.