AVG Error: Update Process Failed

Hi!
My friends told me about the AVG Antivirus and then I installed it on my PC. I download the free trial version of this software to try it. There was no installation problem but after a while I found a message telling me that I must update the software. I tried to update it by opening the software GUI and then clicked update and I found an error message appearing as follows.
Update process failed..
Reason: Sorry, you do not have
permission to execute avgupdate."
So what can I do to update this software? It is a very urgent issue to the Antivirus software to complete saving my PC.
Thanks in advance.
