I'm having the same problem in the same situtation.
Using Win7 PRO + UNO r3.
Getting the same error: "D:\0_INSTALL\Arduino\Arduino ERW 1.0.3\hardware\tools\avr\bin\avr-ar: unable to rename 'core.a'; reason: Permission denied".
Yes, I'm using Arduino ERW 1.0.3 but the official Arduino IDE gives me the same error.
The folder has write permissions. I'm logged in as Administrator. I don't have any Anti Virus installed.
Any help would be greatly appreciated!