PC not switching off on clicking shutdown

I have windows XP stand alone PC. All was well till a couple of days back. After working on the PC, it refuses to shutdown on clicking the shutdown button. Although the icons vanish (after almost two full minute) and it appears that the PC is all set to shutdown..it just does’nt. This problem does not occur when I open the computer and switch it off within say 2 minutes, but the moment I work longer, say for more than 15 minutes, rest assured it just refuses to shutoff. Mind you, the cursor still moves on the screen. The icons vanish and only the Wall paper remains on the screen.

I am desperate. please help.

Warm Regards to all

Gopi