If you have OneCare installed and the backup was made with OneCare, no matter if it was the same PC before a reinstall or another PC, you can restore the backed up data via OneCare, using the custom restore option to select a restore from another computer.
If you do not have OneCare installed, you can unzip the files that comprise your Outlook.pst and use a 3rd party tool to rejoin the split file - http://www.freebyte.com/hjsplit/ This utility is freeware and is used by support to deal with issues of restoring a broken or unrecognized backup.
Note that using the OneCare restore feature does require that the backup reside on the same media that it was backed up to. That means if the backup files are on an external disk that was directly connected to a PC, it must be directly connected to a PC and can't be accessed as a Network Share to perform the restore, for example.
-steve
