As kjanx says without further info its hard to help you. There could be any number of reasons for the crashes but I would initially check your temperatures, could be a overheating problem. Do you have the latest drivers? These include motherboard or chipset, graphics and sound. Faulty memory is also a possibility... I could go on.
So please let us know what your actually doing when the crash happens, any 'error' messages that acompany it and some information on the hardware your using..