IPhone :: Find And Retrieve The Backup Before The Upload?

i'm trying to get my iphone synched with my home computer. I had many contacts entered directly on the iphone and many more synched from my work computer. I retired and now need to synch both. I got the work contacts into my home outlook and synched them into the phone but in doing this I lost the contacts that had been entered directly. Now I need to get the lost contacts from the backup and into my outlook. before the upload of the work contacts i had synched with itunes which apparently makes a backup. i have subsequently synched with itunes. how do i find and retrieve the backup before the upload?

Info:
iPhone 3G, Windows XP Pro