I'd go for the extra hard drive and really you'll be able to sleep better. Try to explain the scenario you had when you still did not know what to do and wanted to recover the default backup from your HP laptop(THIS HELP!Virus redirects to spam sites and other stuff
). Then tell them the importance of the files and setup you have and if it weren't for the recovery you would have to purchase a Windows 7 CD or availed of the HP support and that also costs money.
Parents re always like that because they want to teach they kids the importance of value. Through proper explanation they will see that you "value" your laptop through a good backup strategy.