What Does "HTTP 403 Forbidden" Mean?
When your computer (the client) attempts to access a web page on another computer (the server), this is known as a request. The protocol in which this request is made is called HTTP (Hypertext Transfer Protocol). The client sends a request message, and the server sends back a response message. The first line of the response provides a numerical status code and a textual "reason phrase." "HTTP 403 Forbidden" is one of these status codes.
All status codes beginning with a "4" indicate errors on the client side, meaning the requesting computer. A common one is the 404, in which the client requests a resource that does not exist. The 403 is a little trickier -- in this case, the request by the client is technically correct (that is, it was understandable by the server), but the server refuses to respond. The HTTP protocol allows servers to give a 403 response without any explanation necessary.
Directory Browsing Forbidden
Websites are made up of files stored in folders, just like on your computer hard drive. Because these files are hyper-linked together, there is no need to browse the directory folders. If the client makes a request to browse a folder, and the sever does not allow this, a 403 message will be returned to the client. This is the
most common cause of a 403 error. Sometimes, but not always, directory browsing requests can be identified by a / (forward slash) rather than a file name such as .html or .php.
If you receive a 403 when browsing a site that you can normally access, such as this site, or a site which other computers can access without problem, it is possible that your machine has been infected with malware. A malware program might attempt to crawl the directory structure of the sites you visit and when the server recognizes this, it blocks the malware program -- and you -- at the same time. In this case, you will need to run antivirus software to remove the malware or have your computer professionally repaired.
If you have no malware problems and encounter a 403, there may be several causes. You may be attempting to access a network's internal page without being logged into that network or the administrator may not have set the correct settings for that page. Log in and contact the network admin. Your IP address may also be blocked -- try accessing the resource through a proxy or try again later if you do not have a static IP. Finally, try adding "/index.html" or "/index.htm" to the address, for example, "http://example.com/index.html ."Source: ehow.com