arduino uno program upload exception (ser_open() & stk500_getsync())

I cannot upload program to arduino after the last upload time, but Serial Monitor is displaying previous program result.
There have two different error message between arduino-1.0.5-r2 and arduino-1.5.6-r2 (arduino IDE).
p.s. i do not install ftdi usb driver.
What can i do now?
Also, i want to ask this "avrdude stk500_getsync() not in sync resp=0x30" is arduino problem or PC problem?

Driver Manager
avrdude stk500_getsync() not in sync resp=0x30


Sketch uses 2,550 bytes (7%) of program storage space. Maximum is 32,256 bytes.
Global variables use 184 bytes (8%) of dynamic memory, leaving 1,864 bytes for local variables. Maximum is 2,048 bytes.
avrdude: ser_open(): can't open device "\\.\COM4": The system cannot find the file specified.
	at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(
	at Source)
Caused by: Problem uploading to board.  See for suggestions.
	at cc.arduino.packages.Uploader.executeUploadCommand(
	at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(
	... 5 more

hey did you manage to solve this problem? i have the same problem but did not manage to fix it

try to use another arduino

yes i had a second one and it worked .... does that mean that the first one does not work any more?