My name isXXXXX a licensed attorney. Glad to try and help out.
Sure sorry for your circumstances, truly. My heart goes out to you.
Accordingly, I'm pleased to share some good news with you, as follows. Nothing you've described is one bit unlawful, period. Rather, you've been the unfortunate victim of a scam. The notice you received did not come from the U.S. Department of Justice or any other bona fide law enforcement agency. It's an attempt at extortion, and it (not the web browsing you've described) is the crime here.
This is an excellent overview:
United States Department of Justice Virus Removal
I can't tell you how may times I've received similar inquiries, almost always quoting the $300 figure, so I hope this affords you some peace of mind.
If you have a follow-up question or need clarification, please just say the word by using "reply" to reach me.
I truly hope all works out for you.