Your not providing enough information for anyone to really help. Please list the make and model of the components (Motherboard, RAM, Hard Drive, etc).
Do you notice any certain pattern to the crashing (ie. loading up a game, loading up a website, just turning the computer on, etc)?
No blue screen, so what does it do exactly when it crashes? Does it reboot / go to black screen / completely turn itself off / etc?
Did you overclock anything (either from within the BIOS or in Windows)?
What have you done to try to correct this crashing issue?