Window 7 Pro crash

i wonder can anyone help with my problem.I am running Windows7 Pro 32 Bit.over the past couple of months my machine has crashed to ''computer locked'' a number of times.where normaly you press ctrl,alt,del and ''lock this computer''.has any body had this problem and if poss,what causes this.
thank you

thanks for reply.but not quite what i expected,but will try. thanks again