Recently I moved to a new machine having Windows XP.
When I tried to run ASP.NET applications I got 500 Internal Server Errors. On checking the Event Log found the following entry

The server failed to load application '/LM/W3SVC'.  The error was 'The specified metadata was not found.

The solution was the following

1. Start the DTC Service
2. Run the following three commands from the %windir%\system32\inetsrv directory
   1. rundll32 wamreg.dll, CreateIISPackage
   2. regsvr32 asptxn.dll
   3. iisreset /restart

Once the above commands were given the ASP.NET application started working.