Windows 7 here.
Had it my whole life so far.
I use Internet Explorer (I find it faster than Google Chrome for some inexplicable reason), just a hint there. To keep yourself virus-free, try to avoid clicking on adds and download stuff from trusted sites only, like Mediafire, Gamefront, etc. Also, I've got no anti-virus software WHATSOEVER, and I've never gotten a virus in my life (used BiteDefender to check - did a whole computer scan and nothing was found).
Goes to show you what a little caution does.
Run smoothly? I'm using a laptop, so there's no help from me there.
It's noe like I'm going to go and rip it apart to put in a small graphics card that doesn't offer much of an improvement.