Hi,
If this computer was running iChat in earlier OS versions it is likely you have a Saved folder called iChats in ~/Documents
That is Open your Users Folder (Little House icon) and the Documents and the iChats folder is in there.
Messages in Mountain Lion store Saved Chats in a folder called Archive in ~/Library/Messages
This Library is hidden in Lion onwards so use the Finder > Go Menu whilst holding down the ALT key and selecting the Library that appears in the List.
Navigate to Messages/Archive they will be Saved by Date and Time.
In the Messages folder are also some chat.db items.
These also hold the iMessage "history" for each conversation you have had since installing Mountain Lion.
To be clear:-
If you are Saving Chats both IM chats via AIM, Jabber, Yahoo and even Bonjour are saved in the Archive as are the iMessages conversations.
The iMessages ones are also in the chat.db items and it is these that allow you to "scroll back" though an iMessages conversation - at least to the date you Installed Mountain Lion.
Older iChat saved Chats may well be still in the iChats folder in Documents if you used iChat before upgrading to Mountain Lion.

9:04 pm Wednesday; April 2, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad