Hi Lyd,
The INF folder is a hidden folder. In Windows Explorer, you need to go to Tools, Folder Options, View. Change the settings so that you can see hidden files and folders and protected operating system files. Additionally, you can also just click Start, Run and enter C:\Windows\Inf and go straight to the folder.
Also check out Clem's recommendations on the DCOM permissions. To access these go to All Programs, Administrative Tools, Component Services. Once Component Services is open, expand the Component Services, Computers, My Computer, DCOM Config branch. Right click on Defrag NTFS and Defrag FAT engine. Select Properties. Go to the Security tab. In Launch and Activate Permissions, select Customize and click Edit. Ensure that Administrators is listed and has full control. You can also add your specific username and give it full control as well.