Access is denied. Check that the Default Content Access Account has access to this content

This is one of most common issue reported by people on the internet. I also came across this issue and able to solve after doing deep dive into MOSS search.
If you are facing this issue, look for following:

1. Check default content account have read permission to the site. Go to Central Admin-> Application Management -> Policy for web application. Confirm that the default content account have full read permission to the web application

2. Try changing the host file to crawl the local server to eliminate network issues. This is just for testing and should not be done in live environment

3. Make sure none of Web front end server have spaces in their custom headers. Seems crawler don't like spaces in custom headers

4. Finally, this took us a weekend to note. Someone removed the following site permissions "Use Remote Interfaces" and "Use Client Integration Features" to stop developer to connect to MOSS site using SharePoint Designer. This BROKE THE SEARCH CRAWL. So, make sure for your application these permission are enabled. Go to Central Admin-> Application Management -> User Permissions for Web Application to verify this.