Your system denied write access to the Hosts file HijackThis

Suspecting an infection on my Windows 7 Ultimate machine I installed a trail version of eTrust Anti Virus.
Performed a scan on the complete machine, but to my surprise it did not find anything at all.
I ran the HijackThis on my PC and it comes up with access denied on my host file
For some reason your system denied write access to the Hosts file. If any hijacked domains are in this file, HijackThis may NOT be able to fix this.
If that happens, you need to edit the file yourself. To do this, click Start, Run and type:
Notepad C:WindowsSystem32driversetchosts and presenter. Find the line(s) HijackThis reports and delete them.
Save the file as ‘hosts.’ (with quotes), and reboot. For Vista:simply, exit HijackThis, right click on the HijackThis icon, choose ‘Run as administrator’.
Now I am not sure how to detect if my machine has an infection or not.
Anyone has any ideas?
