I was placed on ARD a couple months ago. In PA ARD means that your record will be expunged after completing the requirements. In the court process I did not admit any guilt nor did any facts of the event go on record in the hearing. Everything involved in the case is destroyed after completion.
I won a contest but in order to receive the prize one of the requirements is that I cannot be on CRIMINAL PROBATION (this was clearly spelled out in the fine print). I meet all other requirements for the contest. Will being on ARD prevent me from receiving the prize?

Normally no it is not considered probation but a presentence diversion program. The issue will be if you don't complete the requirements to have it expunged. Now please understand that everything is not destroyed, that just does not happen in today's computer world. First the arrest and the expunged notice will always be on your NCIC report, that police can see this as can anyone doing a national security search but it is not public record any longer.. The court dates, court calendars are not destroyed. So there is always a paper trail.