Well, I think you should definitely upgrade the OS (Win7 Home Premium or Win8). Windows 8 has a decent antivirus built in so that can save you some money, otherwise you may want to get a good third party antivirus like Kaspersky or Trend-Micro, or get AVG antivirus for free.
As for the keyboard and mouse it really comes down to preference. Do you want a mechanical keyboard with all the LED backlighting and macro keys, or do you want something simpler? It's up to you much are you are willing to spend on a mouse and keyboard.