Cousin's computer won't start up.

Hello,
When my cousin's computer gets to the welcome screen upon loading up it sits there for about 15 minutes, then, it brings him to the screen to choose an account and when I clicked the button for his name it shows his wallpaper briefly, then just goes back to the login screen.

Any idea what is wrong with it?

His is a Windows XP

It's a dell and the only identification I found said model: DCSLA