I tried to install a new memory module to my sons PC but when I switched the machine back on nothing happened therefore I took it back out again. I now have the original 3 modules back in place and when i power up the nmachine emits one beep and then switches itself back off. I was wondering is there a wrong way of reinserting the memory sticks and what would be the best solution to ascertain whether it is the memory or something else that is causing the machine not to start.

you might of accidently damaged one of the delicate tracks on the motherboard, make sure you used the correct type of ram and ideally the same make of ram. if all else fails take out all the ram and place them in one at a time untill you find the guilty stick thats causing the error.

Try clearing the CMOS. There is a jumper on the motherboard mark clr/CMOS or something similar. You need to remove the jumper from the 2 pins it is on and place it in the other position for a minute. Perform this operation while the computer is unplugged. If you can't find the jumper, you can remove the CMOS battery. It looks like an oversized watch battery. If you need to do the battery trick you will need to keep the battery out for some time, maybe even overnight. This will set the BIOS back to the default settings. You may need to enter the BIOS screens to reset some selections, if everything doesn't work as before. If you get the computer to work again, I suggest that you download a free utility called AIDA32. Run this program and it will tell you what kind of memory you currently have. You may have error correction memory or slower memory that the new, I guessing here. AIDA32 will tell you. Get it at AIDA.hu If the machine still won't boot after clearing the CMOS, then try removing memory and insert one stick only in the first slot. If the machine then boots, alternate one at a time to determine if all memory is good. You need to look at a manual for that motherboard. There are too many variables to guess. Some motherboards only allow all memory slots to be filled under certain conditions. AIDA32 will give you the model of your MB. Go to the manufacturers website and download a PDF of the manual. You will need adobe actrobat to read it. Adobe actrobat is free all over.

I have now got the PC to power up but monitor won't work.the monitor light is on but just a black screen. I have reinstalled the 3 original memory modules that was in the PC. I have checked that the video card is secure but still a black screen. Any other suggestions would be greatly welcome

Did you try to do what I suggested?

No I didn't get around to doing that I just double checked that all the meomory modules were sitting correctly in their slots and then restarted the machine. It powers up fine now after emitting one beep but just a black screen I have just tried a monitor self test and this was OK so i am wondering now if I have inadvertently done something to the AGP graphics card.

Try booting into safe mode (F8) and going to device manager. Remove the display adapter and reboot. should come up with a default VGA screen You then need to install the drivers for your graphics card.