Viruses and worms are a non-issue for Macs right now. Spyware is probably non-issue, too. These are things that you only realy have to worry about if your running Windows.
However security is a concern, OS X isn't one of the most secure OSes out their by a LONG shot. Keep it behind a firewall and keep it up to date thru Apple's built-in update tools and you should be fine.
As for the internet slowdown, not sure. She could be hacked, it's always a possibility, but it's unlikely. Much more likely is that there is either something wrong with some part of the OS that inadvertantly happenned, or that your ISP has degraded in service quality.
Like maybe try some ping tests and such. See if your getting packet loss (that's a problem with my ISP sometimes.) Also if you type a DNS name in there (like a
www.blahblhablah.com) and it just sits for a while it could just be that it's having a hard time resolving the name into a ip number. You can try and do something like use a alternate DNS server or something like that.
Or maybe it's a configuration problem with Safari, try maybe IE 5.0 (bleh) for a bit, or firefox and see if it acts any faster.
check out here for some OS X maintainance tip. I usually run a permissions repair time to time, and that helps keep everything stable and fast by quite a bit sometimes.
