You could take a look in the documentation which came with the machine and check which F key you press after you switch the machine on. It may well show the keys you can press on one of the initial screens - its probably F2 or F12. Then follow the instructions to reset the time.
XPS 420, 8GB, Windows 7 Home Premium SP1 64 bit
it's a Dell Dimension 1100 windows xp. So how do I get to the system setup [BIOS] if i can't log in?
Boot the system and when the blue Dell logo appears, press F2 immediately.