pure or 2012?

Make one your primary AV program as you
should run only one at a time.
THW

I don't use either one..
Am using free MSE with Windows Firewall along with MBAM and SAS(both free spyware protections).

This works for me but may not work for you.
THW

First of all i would like to say that when i tried kaspersky last time a couple of year ago i found it resource hungry i am not sure what version 2012 is like. That being said i would advice you too use the trial period to see how it performs because each pc is different in terms of hardware and software environment so something lightning fast for me might be slow for you so its always best to try the AV before purchasing it. Kaspersky pur and kaspersky 2012 differ in terms of functionality so look carefully in terms of what you need. The core virus detection engine and update frequency would be same so look at the chart and decide what you need. For example pure has two more function namely password manager and data backup. IMO both are use less function because you can use keepass or lastpass as password manager (both free) and also use paragon back up and recovery or macrium reflect to back up your data (both have free version). Now its up to you how much you want to spend.
http://www.kaspersky.com/compare

PS: you are still using xp service pack 2 please update your OS its really important to keep all your software as well as the OS up to date for security reason.