problem uploading programs

Hi everyone

please guide me to solve this problem. Used Arduino Uno (Windows 7 enterprise(SP1)) for a week and whwn i tried today its not uploading programs now(which it did till yesterday) having changed nothing . I tried different programs, tried resetting arduino, restarted......NOTHING WORKED!!!

here is error message in which shows up in red:-

avrdude: verification error, first mismatch at byte 0x0000 0xff != 0x0c avrdude: verification error; content mismatch

And status shows uploading..but finally dosent.

I've tried mailing but got no reply. Can anyone help?