1. Build your Windows machine in a virtual machine. (VirtualBox is good.)

2. Run either Linux (Ubuntu is good) or another copy of Windows as your base OS, and run VirtualBox (or other) for hosting your Windows guest.

3. Immediately after creating your Windows VM, back up the (clean) VM to a safe place. Make it read-only. You'll need it to re-install, like Ghost -- which you won't have to purchase.

4. If Windows is your base OS, uninstall all web browsers, etc. so that you are NOT tempted to access the Internet from your base OS. (The idea is to keep your base OS free of viruses...)

5. If/when your Windows VM gets infected, quarantine it and re-install your read-only backup VM.

Note that you can spare yourself Step #4 and purchasing a second copy of Windows by using Linux as your base OS. You can surf the net with confidence from Linux. (At least for now... :^)

You can also also use a "second instance" of your backed-up VM to "test drive" a website / download / Facebook page / application -- if it brings in a nasty, you just delete the VM afterwards, and go back to "business as usual".