I'd be doubtful that this can be done successfully for less than starting over. You will need a new PS as well. The P4 will need plenty more power and cooling than the P3 and the case may not be able to provide adequate air flow. Because this is a brand name PC, chances of success are lessened by the fact that these are proprietary boxes that don't always accept standard replacement parts. I hate to discourage but you'd probably do better to replace the whole thing.
I want to upgrade my pc.I have HP VECTRA VL400 PIII. If I buy PIV motherboard or celeron upgrade version, will it work? what processor i have to buy then and what more things i need to change?