Help Please!

A few months ago I had recently got cable and internet hooked up at my house. So I tried to connect my laptop to the internet, but it refused to work. I was told by what I thought was a reliable source, that Mcafee had an issue (blocking my internet) and needed an update to fix it. I was told to boot the laptop in Safe Mode, go online and download and install the update. So I did just that that and in my attempt to restart the computer, my computer went crazy and spent about five minutes showing a bunch of messages on a black screen. When I attempted to reboot my computer, it will only show (right after Dell Animation)  Error 15.... nothing more, nothing less, just Error 15.