What kind of bot? There are at least tree:
What you need to be done will influence how you build your bot.
You can read more there: http://en.wikipedia.org/wiki/Wikipedia:Bots
That's hard to make because you'd have to either hardcode phrases so that even if someone says hi instead of hello, if hi isn't harcoded, there will be no response. The other way, which is a complete pain to do, is creating a neural network which understands the questions being asked for responses. Not impossible, but very hard.
If you can find where the bot was, you might want to take a look at it and see if it's a freely available piece of software, it might be easy to implement, but as Kangaroopower said, you'd have to take a lot of things into consideration.
You could fake it with cleverly written heuristics, instead of using an AI
Take a look at the ELIZA program (the fake psychologist)
What do you think?