Here is 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.
The computer began to wail like an ambulance is a main clause. A virus had immobilized the hard drive is another main clause. These two main clauses have been joined with only a comma, thus creating a comma splice, a major error in sentence structure.
To fix the sentence above, you could use a semicolon between paper and unfortunately.