|To clarify the situation: this is a desktop system - not a laptop?|
Presuming it is desktop, and thus the monitor is obviously an external one, is the monitor powered up and its power light showing "on"?
Even if only very briefly do you see anything on the screen?
Can you access the bios settings - sometime rederred to as setup - when the system starts to boot?
How are you starting the clean installation?
Do you have a proper XP setup disk, and if so what type, and where from?