Can't positively answer your question but have you looked at the file extensions (the bit after the dot in the file name). If the old files have different extensions to the files created by the new programme try making a copy of one of the old files, change the file extension to match the new programme using the "Rename" function and tyry and open that. Failing that I think I'ld drop Broderbund an e-mail click here.
Got the address off the Broderbund "PrintMaster Platinum" I have here. The contact bit is at the bottom right hand side of the web page.
It does say on the software I have it is compatable with Win 98, 2000,Me or XP so I'm a little surprised your old files don't work.