hi mate
this is a common and annoying problem and is normally due to a hardware issue,and this can vary from a overheating processor to incompatible memory module
to discover the reason for the "serious error" got to start>control panel>administrive tools>services>then look down the list of services for today and you will see a entry for "serious error" and within that entry you can discover further information on why its happened,however most of the details are quite technical,normally memory adresses and cpu info,which to the average pc user wont mean a thing,but to a microsoft certified technician will give him valuable information on the reason for the error
to stop any further reboots got to control panel>system properties>advanced>start up and recovery>then untick the box marked "automatically restart" and press ok...this will stop any future reboots however you still need to discover the reason for the initiall reboots
for starters i would buy a can of "air duster"(compressed air) and clean your cpu-fan and all other fans in your case,and then clean other hardware including hard drive and optical drives,a clean p.c will work much better than a dirty one
also run both anti-virus and spyware scans to check for malware infection as this can cause similar problems
finally windows has a knack of getting bogged down with old files and registry entries which can cause errors and reboots if not cleaned on a regular basis,and the best program for such task is "advanced windowscare personal" from http://www.iobit.com/advancedwindowscareper.html
this is a one-click solution for cleaning your p.c and also removes spyware and registry errors,plus temporary internet files and cookies and much more
using this every other day will keep your p.c,s hard drive free of clutter than can in itself cause errors and other problems
any more advice let me know
good luck mate !