The main cause of this issue is that your website has inherited "deny" permissions from the parent folders that contain it.

Resolution:

• Locate the root folder for your website in windows explorer. The default location for a v4 install is:
  c:\Inetpub\wwwroot\Configur8orv4
• Right-click on the Configur8orv4 folder and select "Properties" to load the properties dialogue.
  
• On the properties dialogue, left click on the "Security" tab
• In the list titled "Group or usernames" left click on the "Internet Guest Account (IUSR_" and the permissions will fill out below, under the "Allow" and "Deny" columns.
• If the "Write" permissions are ticked under the "Deny" column (and it is greyed out), continue following these steps:
• Press the "Advanced" button to load up the "Advanced Security Settings for.." sub-dialogue.
• Un-tick the checkbox that is titled "Inherit from parent the permission entries that apply to child objects. Include these with entries explicitly defined here." At this point a "Security" window will pop-up, on this security window press the "Copy" button.
• Finish by pressing the "OK" button on each dialogue. If you see any further "Security" messages asking if you are sure you wish to continue, click "Yes" on them.
• If there are any "Ticks" under the "Deny" column, un-tick them. Don't worry if "Special Permissions" is ticked, that can stay as it is.
• Finally, left-click on the "Launch IIS Process Account (IWAM_" within the list titled "Group or usernames" and un-tick any checked options under the "Deny" column.

Note:
These instructions are for Microsoft Windows XP and Microsoft Windows Server 2003 only. Please look at the help supplied with other operating systems on how to change the same settings.

Having Javascript or Java disabled will also stop you logging in to Configur8or, see the following articles:
I get this error "Java is disabled! Please Enable Java" when attempting to log on to Configur8or? &
I get this error "Javascript is disabled !. Please Enable Javascript"