Viruses are a Windows disease. Also, Linux never needs to be defragmented like Windows does. It's very possible to even have both on the same machine. You don't have to be a guru to split your windows partition, but you do have to be smart enough to ask for help when you need it, and there's a program called 'Partition Magic' if you're completely clueless. Always follow common sense computer knowledge like 'back up your hard drive first' and 'create a boot disk' before you do anything to your system.
Most failures are because of operator error due to lack of reading the instructions first.
Helpful links are provided at the bottom of this page. If you download a free version from the internet, you will need a download manager in case you can't get it all in one shot. The files are quite large and can take literally hours without a cable modem connection. A good download manager will let you resume the download if you lose your connection. It depends on your needs.