Upgrade your version of LabVIEW. LabVIEW 4.0 and it's serial port driver were developed a long time ago and while Win95/98 retained some backwards compatability with the older 16-bit OS's, Win 2K and XP are 32-bit only. The first version of LabVIEW that NI tested with XP was version 6.1 and there have been a huge number of changes made from 4.0 to 6.1, let alone the most current version.
As I indicated we are having problems reading the original code on zip disks. Upgrading Labview basically means starting from scratch. We are trying our best to avoid that expense, if possible just because of XP.
OK - This is a long shot, but if you used the native serial vi's in the original program then you may be able to use a newer version of the serial driver (serpdrv).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.
I've tried both the compatibility mode by checking the win 95 and win 98 boxes with no success. I will review to see if there are advanced settings. The other servpdrv files did not work at all.. no communication.