"PORT COMn is already in use" when uploading an sketch

I know this is a common problem and I've been googling it a lot but still can't find a solution.
It happens to me almost daily, and the problem appearing and disappearing seems to be something totally random.
The board is a Mega2560, OS Windows 7 Ultimate 64, UAC is disabled, IDE version 1.0.5.
I usually try restarting the PC, unplugging/replugging the board, disabling AV software, reinstalling the port, sometimes some of that works, sometimes doesn't.

EDIT: Sometimes I get this error instead

avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: ser_send(): write error: sorry no info avail

Thanks in advance for any help or advice.

I found -this time at least- the cable was the problem.
Wonder how it can fail, bought it yesterday and was working fine at first.