It may depend on the format of your PST files.
PST files are PST files. If X1 can handle one that works in 2007, then it should handle on in the same format from Outlook 2003.
Outlook 2003 began the switch to Unicode files. If you are on Unicode, you could just try X1 Search 8 to see if it indexes.
Actually, I am not sure that X1 Search 8 can't handle the older format. In which case you may still be in luck.
PST files are PST files. If X1 can handle one that works in 2007, then it should handle on in the same format from Outlook 2003.
Outlook 2003 began the switch to Unicode files. If you are on Unicode, you could just try X1 Search 8 to see if it indexes.
Actually, I am not sure that X1 Search 8 can't handle the older format. In which case you may still be in luck.