OK if your starting out at web design and you want to use a template I suggest you have a look at the coding and try to work out what does what. First look at the HTML then work out what styles are been used to make it look the way it is (css). Then try editing the css styles and see what effect it has on the site.Once you start getting the hang of things you will start to see how things are structured and after time it will come to you. Know one has ever taught me how do code but from looking at other peoples and getting an understanding I can now open notepad and write valid CSS & HTML with no problems, obviously when it come to browser compatibility there is always some tweaks to make. Never write a whole page then test it in various browsers, test it as you go along.
I tend just to use the code view in dreamweaver then have the page open in a browser refreshing every time I add some code to see if it looks fine. I test all my sites in IE3-7, Firefox, Netscape, Safari and all are valid XHTML & CSS.
I used to be able to sit most of the day in front of a PC but as the days go by this time is reducing with the strain on my eyes and pains in my wrist. I do try if I am going to be on the PC most of the day to have regular breaks.
The above post is correct in some sense but you can also get viruses from other sources (CD's, Flash Drives etc) especially if they have been used on a computer that is connected to the internet.