how to transfer iPhone backups between PC's

ON THE OLD PC
first of all, you must show HIDDEN files and folders
do this through control panel > folder options, then click the view tab

windows 7 location -- C:\Users\UserName\AppData\Roaming\Apple Computer\MobileSync\Backup
windows xp location -- C:\Documents and Settings\UserName\Application Data\Apple Computer\MobileSync\Backup
copy the entire folder name in the backup directory to a usb flash drive or cd-r.  example - d76015282d585f8548b2a3ffc5c4d0ff435cb3b8

ON THE NEW PC
ensure itunes is closed
paste the folder in the same location where it was copied from
open itunes
click edit > preferences and click on the devices tab
you should see the backup there