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....


Well first off to service any computer you need a few things, the correct tools, and an antistatic mat to place the parts on. Make sure when you buy an antistatic mat, that it comes with a grounding cable, and make sure you ground the cable properly according to the directions.
Presario 700's require a T9 torx driver to use, these can be picked up for a few bucks at any Autozone.
Once you have those two things you are ready to begin. First, remove the screws in the bottom of the computer, then pry up the panel that contains all the "smart button" (your power button, email, etc). Then lift out your keyboard and disengage the cable that runs to the motherboard. Then remove the screws on the heatsink in order from 1 to 4 and carefully pull up the heatsink and detach the fan cable. After you have accomplished this you should be able to slide out the CD drive.
Now, you should be able to see the back of the DC injack where the power cord attaches to the computer. If it is damaged, there is your problem, and I can further coach you on how to fix it. If you cannot tell for sure the only way to get a better view of it is to remove your LCD.