Answer: A) Your user inadvertently downloaded a macro virus using IM
Step-by-step explanation:
As the user has downloaded a file from a prospective client ad the file was account.doc, but since then the system of the user has not been behaving normally so the most possible reason could have been the user has inadvertently downloaded a micro virus using the IM. These micro virus are invisible programs which can be transferred inadvertently from one user to another another. They can be transported over the network.
Option b is not correct as the user has not download a rootkit as it would require special privileges.
Option c is not correct as the user upon changing a setting in the system can reset it.
Option d is not correct as using the IM is not unstable to the system.
So, the correct option is A.