Assuming you do not have the username and password, and the data is UNencrypted (if it is, you have a problem!) then what I would do myself is simply:
Remove hard drive
Place drive in a spare drive bay or enclosure attached to the desktop/workstation where you want to copy the data
Boot with any Linux Live CD and copy the data using the file manager
However there are MANY things that can complicate this task, such as encrypted data, RAID, hardware failure, etc. so why not just hire a consultant/temp for a half day?