I can only assume your ISDN card is too old to use an IP connection.
I did actually get a serial conection to work using XP when it first came out. it entailed taking a win2k machine that had a working IMAT serial connection set up. then you used the "file and setting transfer wizard" from XP and transfered the "internet" settings. before you imported it into the xp box, you had to set up your serial cable between two computer modem.
I did this probably 6 times to various computers. had to redo my win2k files when SP2 came out. never did try it on SP3. when Vista came along, I created a vmware workstation with a win2k OS and have used that ever since.
still run into the occaisional PRI card that is too old to access via IP.
one thing I have done on a win2k ISDN gateway is to copy the files out to a floppy and then copy them into the appropriate folder on the IMAT machine and access them that way. you should be able to do that by following the instructions to manually upgrade it to get access to the files. it would take it out of service though. That is assuming you only need to access it in order to upgrade it.