I will tell you the same thing I told ploppy this is extremely common on the 700's. I am almost sure this is your problem.
"Ok, first off I have an old compaq presario 700 and I have serviced multiple others. The power supply is integrated into the motherboard so buying a new one is out of the question. One of the most common problems with the 700 is the internal port into which the power cord connects often breaks. If you plug your cord into the back of your computer and can wiggle it around, the port is probably broken. The best way to check is to take out the keyboard and your cd drive and to look behind the cd drive at the port. The computer I am using the port completely broke off but I have been able to fix it up a little, just enough so it runs. Let me know if that is the problem and I will try to help you more."
I have more or less the same problem but i am not willing to have the Presario serviced from HP. I prefer trying it on my own as long as someone help me.
Could you please explain the procedure so that i can check if i can follow your guidance. After all I am going to need to purchase the appropriate screwdrivers e.t.c. if i understand what you will say....