XPS 200 with Sponantaneous Shut Down Issues

I have an XPS 200 desktop with Windows XP Media Center 2005 installed on it.  I've had the computer for a few years and it has worked fine until last year I started getting blue screen errors that didn't allow me to access my computer.  I took the computer to a shop which fixed the problem but since then the fan has been extremely loud.  When the computer is on Stand By it is silent, but as soon as I turn it on the fan starts running loudly.  Also, every once in a while it used to spontaneously shut down without warning and the power button on the front would blink orange.  Now, however, it is shutting off two or three times a day without warning and I can't find anything wrong when I scan for viruses or try to follow trouble-shooting suggestions on the Dell website.  Please help!!