Basically, almost no virus written for windoze will run on Linux. I get them via email and sometimes on IRC, at least I think that is where they come from, and have never had one to actually work. Remember, except for emulation, windoze programs will not work on Linux.
I have read there are only a few , dozen or so, "viruses" for Linux. If you have a up to date kernel and upgraded software, they are fixed. That is the beauty and strength of Linux, bugs get fixed very quickly and they are usually public and a work around is available. You don't have to wait for a fix if you can program. You can get the source code, fix it, compile it, install it and your done. Try doing that with windoze. You don't get source code for windoze XP.
As usual, denial of service applies to any system, Linux or windoze even a Mac. It really has nothing to do with the OS. If you have a system and a pipe big enough, a DOS attack would not work.
You should never connect to the net as root unless you are SURE of where you are going. I do this when I update portage, sync, and for everything else, I am dale. I have been using Linux for quite a while. I do not use windoze at all. I have never bought windoze either. NEVER install software unless you are sure where it came from.
I saw a while back where there was a hole in Linux kernel. By the time Dan Rather was on the news making a big deal of it, the fix was available. We didn't even have to wait for the next windoze update.
Nutshell: never run as root, be carefull if you do. Be carefull with what you install and its source. Keep your system up to date.