Answer: A storm caused a power failure, so Peter started the new generator.
A computer virus has totally wiped out my had drive because I did not renew my subscription to my virus protection program
Explanation: for the first statement, the main clause is 'A power failure' as it is drawing attention to the action carried out by Peter. Joining the 2 statements was only possible if 'so' is added, as it shows exactly the reason why Peter started the new generator.
For the second statement, computer virus is the main clause. 'because' is a word showing what had caused an event to occur, so it is the best joiner for the 2nd question