In short, the likelyhood that the new CPU will just plug in and work is about zero. The machine in question has a BIOS that must support the new CPU, the power supply on the machine and motherboard must supply enough power for the new CPU and all the parts must fit.
Compaq makes a lot of ready to use machines with little room or support for such an upgrade.
You, as the upgrader would know arcane things such as FSB speeds, socket types, heatsink size and clearances and more. If not, then it's unlikely this will work.
Bob
I have an old Compaq Presario that I like but it is slow - Intel Celeron processoe 431 MHz. I see where I can buy an Intel Pentium 3, 1.8 for about a hundred bucks. My question is "How feasible, how difficult is it to change out a processor?". Is it just a matter of taking the old one out and slappin' the new one in or is more complicated than that? Thanks

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic