clRFU.exe with Windows 7

I have Python Program on PC side which calls clRFU.exe to program the Rabbit 3000 on RCM3700 board. It works fine on XP machine where I work normally. But when I install my application on Windows 7 and call clRFU.exe then sometimes it fails to connect to circuit through serial port and sometimes it connects but does not return after programming finish(i.e. just stays at the end where programming finish)
Here is my statement for that:

if (os.system( 'clRFU.exe "%s" -s %d:115200 -cl coldload.bin -pb pilot.bin -fi flash.ini -v' % ( fileName, int(std.config['comport']) ) )) == 0 :

Please let me know, why does it fail and what can be the solution.

I am using Dynamic C 9.62.
Thanks in advance.