How to remove the top cover of compaq deskpro p3?

Power supply First to check. they act really weird when faulting.

Look at the back of the computer. One side panel removes. (Access Panel) There are two Thumbscrews to loosen on the Access Panel, then the panel is lifted off, or brought straight out.

http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?contentType=SupportManual&lang=en&cc=us&docIndexId=64180&taskId=135&prodTypeId=12454&prodSeriesId=96266#2

On this HP Support page click on -

Deskpro EN Series of Personal Computers Maintenance and Service Guide

Regards,
joecoolvette

The power supply is faulty and needs to be replaced, undo the screws holding the top cover on, pull the cover up and away from the from the front, the power supply will be at the back on the right, there are four screws holding the power supply in place, they are at the back of the computer case, undo these and the power supply should come out, make a note of where the power connectors go before removing, take the power supply to your local pc store, as most power supplies are standered the compaq power supplies maybe slightly smaller in size, so your local store may have a replacement or you may have to order one straight from compaq. hope this helps. cheers.

This isn't a sure fix by any means, but...
You could try unplugging the unit from the wall socket and opening the case. Touch a metal component inside the case for a few seconds to equalize static charge between yourself and your machine.
Next, remove the battery on the motherboard, and wait 5-15 minutes before reinstalling it. What this will accomplish is to reset the computers' BIOS (Basic Input Output System) settings to the defaults for the motherboard, which may let the machine figure out what processor you installed.
If it works, your machine will boot up, and you'll get an error message telling you that there is a checksum error or simply that a BIOS setting is wrong. You'll have to read the message and press the appropriate key to get into BIOS, then reset the date and time.
I hope this helps in some small way- It's rough attempting to set up a machine if you have no visual output to let you know what's going on.
If my solution gets you up and running, please let everyone know by writing a testimonial.

See: http://www.tomshardware.com/forum/12003-2-need-model-deskpro

for details about your system.

To upgrade from a Pentium 3 to a Pentium 4 processor, you would have to replace the motherboard. The Pentium 4 processor has more pins on it than the Pentium 3 and would not sit in a Pentium 3 motherboard.

its may ur cmos battery problem u can replace ur cmos battery and check that prolem

Use the following steps to open the case: WARNING: The edges of metal panels can cut skin. Be careful not to slide skin along any interior metal edge of the computer. CAUTION: This product contains components that can be damaged by electrostatic discharge (ESD). To reduce the chance of ESD damage, work over a non-carpeted floor, use a static dissipative work surface (like a conductive foam pad), and wear an ESD wrist strap connected to a grounded surface.

1. Unplug all cables except for the power. Write down each cable location if you do not already know where it connects.
2. Unplug the power cable and with the power cable removed press the Power button on the front of the PC.
3. Remove the screws that attach the side panel to the computer. Do not remove other screws, such as the power supply screws, that are also located on the back of the computer.
4. Slide the left-side panel towards the rear of the PC to remove it.
5. If the front of the drive is blocked by an extra door that is part of the case, remove the front cover. You may have to remove the opposite side panel in order to access the front cover. Press in on the tabs that retain the front cover, there should three to four on each side of the cover and gently pull forward. It helps to have a friend help with this process. Carefully detach any cables that may also be attached to the front cover.

Dimensions: 254mm = 10.0" x 218mm = 8.5 inches. This is the current size of motherboard in the Compaq.
http://stason.org/TULARC/pc/motherboards/C/COMPAQ-COMPUTER-CORPORATION-Pentium-DESKPRO-2000-P.html

I read the thread over at computing.net just now. I'm having trouble locating a picture of the internals of the Deskpro 4000 online but I know Compaq did some pretty non-standard things back in the day.
Yea, try upgrading the processor - it's not going to cost much to try. Much as WebsWonder commented, I've been throwing out everything below 2ghz at work.
The 2mb S3 Graphics was great for its day. Look around on eBay and find a PCI interface card with 8 meg of ram or better. An ATI 7000 series would be good n cheap but even a Rage 128 would be an improvement over what you have now.
I honestly don't know if upgrading to 333 mhz is going to produce smooth video for you. I upgraded my son's Compaq Armada 1750 from a P-II 333 to a P-III 500, bumped the memory up to the max 312, and installed a 7200 rpm hard drive. It streams video just fine on XP Home.