My computer is about 2 years old, and now we a turn it on it will go to the XP welcome screen, then when I click on the user account, it will sound like it is loading up but all that will come up is the desktop wallpaper. None of the start menus and icons will show up. I have restarted it several times with no change. Occassionally it will have a box pop up that says error in a .dll file within the C:/Windows. I am unsure the excact name of the file. Any Suggestions?

When it shows the wallpaper try pressing Ctrl+Alt+Delete and then go to run
in there type in explorer.exe
see if windows will start up properly by doing that