Crazy windows malfunctions...need ideas

Alright so, I have a client who runs a windows XP home system. I have been trying to troubleshoot a program called Winfax V10.0. The problem is when you open it and goto the contacts and try to add a new one a info page comes up on the top have is contact information and on the bottom half is space for up to 4 #'s. you can fill in the personal info fine but when you click on ANY field in the phone # area the program completely crashes.... no error... no effect. The same things happens wether it is a new entry or editing a current entry. They have a identical computer running the same program with no problems. So far I backed up the database containing his phone list. As of now this is what I have tried. I have reinstalled over the original program, failed, I have completely removed the program from the system *INCLUDING manually going into the registry and deleating everything referencing the program. after a fresh install I took all the files from the other computer with the functional software and overwrote everything in the directory, including shared files. the EXACT same error occurs every time on every attempt. This is obviously something to do with windows itself but a complete rebuild is the last resort. if anyone has any ideas or needs any more info to help them try to figure this one out it would be greatly appreciated