The Vista OS has been known to have compatibility issues when running serial communications programs on multicore processors. This is because it allows multiple cores to work on the same program which can cause synchronization issues. To solve this, use the “Set Affinity” to 1 core. There are various ways to do this (Google “Vista Affinity”). One way change that setting when the program is running is to right-click the application from within the Task Manager, then set the affinity.