Crashing if that's the word?

Please I need help.
Hopefully this is enough information for ‘mm’ to decide if he/she knows what the problem might be.
Case description :
• Case measurements 20.5 cm wide. X 51cm deep [front to back ] 51cm high.
• Has fan housing on top at rear . Top is black in colour
• Front: black inner front and silver outer vertical hung front cover. There is a cut out for the USB sockets and indentation in the cover to enable ease of opening to access the DVD drive DVD drive Samsung Super Writemaster. Fitted about 1/3rd down in front housing working fine when I can get the b***y thing going.
• Sides are sliver panels with black trim.
•
• Operating on Windows XP professional.
• Power lead is grey so I know which plug to pull when it can’t be switched off, which sometimes occurs with this problem I am near describing. Have about six games on it can’t start to count. Have most of my information on a removable hard drive and the rest and progs on the computers hard drive.
• The Fault is :
• It’s been cutting out/crashing on occasions and last week working on project and saved one and partly started another then in comes a fast fan and the screen goes blank and I had to hold the on/off button in to complete shut down, sometimes had to pull the mains plug.
• Now about does the same about 15secs after start up without even reaching the desktop page it goes haywire again.....fast fan........shut down or loss of screen then I have had to unplug because the on/off switch doesn’t always respond.
• Is this clear enough mm?
Thank you in anticipation.