Take a look at the sentence again:
The computer began to wail like an ambulance, startling poor
Robert, who only wanted to retrieve his
paper, unfortunately, a virus had immobilized the hard drive.
Who only wanted to retrieve his paper is a subordinate clause. Although it has a subject, who, and a verb, wanted, it does not express a complete thought. You must attach subordinate clauses to main clauses if you want to avoid writing fragments. In the sentence above, the subordinate clause is attached to the main clause that opens the item, The computer began ....
When a subordinate clause follows a main clause, you usually do not use a comma between the two. Who clauses, however, are a special case. Because Robert is specific, you need to use a comma between Robert and who. See Comma Tip 5 for more information.
Go back to the sentence to try again.