OK - This is a long shot, but if you used the native serial vi's in the original program then you may be able to use a newer version of the serial driver (serpdrv). I've attached the serpdrv file from LabVIEW 6.1.
1) Backup serpdrv from your application directory
2) Copy the attached serpdrv file into your application directory
3) Execute your application and see if it makes a difference.
Unfortunately, without the source code it is extremely difficult to evaulate, let alone debug the program. However, if you do end up having to rewrite your code in LV7 you'll at least have some tools such as the I/O assistant which will make the task much easier than when
it was written in LV4.