sharepoint 2007 showing System Account instead of Username / full name

When we login with our username & password into sharepoint site, it displays as a "System Account" instead of my username or my full name on the top right corner of the website. What ever changes i make inside the site, for example upload a file, post a comment, sharepoint marks it as a System account instead of my name.

To solve try the below
Method 1:
Go to Central Administration / Application Management. Click Policy for Web Application. Select your web app and click your account in the list. In the Edit Users page, clear the Account operates as System checkbox. That should fix this particular problem.
Method 2:
1. On the top navigation bar, click Operations.
2. On the Operations page, in the Security Configuration section, click Service accounts.
3. On the Service Accounts page, in the Credential Management section, under Select the component to update, select Web application pool.
4. In the Web service list, click a Web service.
5. In the Application pool list, click the application pool that you want associated with the Web application.
6. Under Select an account for this component, select one of the following:
Predefined Select this option to use a predefined account, such as the Network Service account or the Local Service account.
Configurable Select this option to specify a different account.
7. Click OK.

Method 3:
To fix this, you must run the following commands:
stsadm -o updatefarmcredentials -identitytype NetworkService
followed by:
iisreset

"HTTP Error 503. The service is unavailable.

When you browse the Central admin page or SharePoint sites you may get "HTTP Error 503. The service is unavailable.

To solve check the below things...
1. Please check SQL server services running in which user account, If user might have changed the password it will happen. In properties --> Log ON --> changes the password.
2. In IIS, check the under sites whether Central admin page or SharePoint site manage web service stop or start. (Should be Started).
3. In IIS, and also check the under Application pools whether Central admin page or SharePoint site task services status. (Should be Started).