I think the simplest way to store and retrieve the data would be by using the COP Instruction. First Create a Storage Array for a length of 121. This gives you 0-120. Then, every minute MOV your current reading into Array[120] and then COP Array[1] to Array[0] For a Length of 120. This will copy 1-120 and Paste results in 0-119. That way your 2 hour old data will always be in word 0. So,Then you just need to Display Array[0] in Wonderware.
Also, you can put this code into a Periodic Task, set to trigger every 120 secs, there's no need to scan a timer every scan.... Give the task the highest priority, and it will trigger exactly every 120 seconds, to the microsecond... (you did say you wanted exactly 2 hour old data....).