Question : Problem: Boot problems

I tried to add some extra memory and an EIDE drive to a working computer with a SATA C: drive
After doing so, the computer would not boot, so I undid what I had done. It still won't boot. Basically, the monitor shows no input signal, so after switch on, I cannot even see the VGA signal from the BIOS setup. It's actually worse than that: I do not hear a system beep or any electronic indication that the computer is even getting into the POST routines, so I don't think it's the video card.
However, the system power comes on, the CPU fan rotates and the Wireless LAN card LEDs start lighting up.
I did remove and replace the CPU (but I am quite sure I got this the right way round and properly re-inserted into its socket). I also removed the RAM (carefully grounded in case you were thinking...), cleaned the contacts and returned to its socket.
I don't think I did anything else. What might I have screwed up and what should I start by replacing/examining to get this computer working again?

AS Rock 800MHz mobo, Intel Pentium IV 3.0 GHz, 0.5GB DDR400 RAM, 120GB SATA HDD, some generic AGP video card.

Answer : Problem: Boot problems

first, i would recheck closely all you did, and redo it. most probably something not installed properly.
if still no go, Here my troubleshooting procedure :
i suggest testing with the minimum setup - disconnect also all peripherals and network cables :
motherboard + cpu + 1 ram stick, video card, power supply
verify that the 4-pin cpu Aux power plug is connected
on boot, do you have a display?
if NO it is one of the connected, swap ram, Power supply, video card or monitor - leaving only motherboard and cpu
if Yes, add devices till the problem shows

you can also check the motherboard for bad capacitors as shown here :   www.badcaps.net