Personally I don't think it would be a viable proposition, even if you could. I doubt very much whether you could put another CPU in. You might be able to add more memory but from experience Compaq uses their own memory (which in this case may be SIMM memory) and is a lot more expensive than generic memory. I also think that a 3gb hard drive would be pushing XP to it limits.
Only my opinion but someone else may think differently.