Processor: Intel Core2Duo E8400 3.00 G Hz
MotherBoard: Intel DG43NB
Memory: ZION 1X2 GB 800 M Hz
PSU: i Ball 400w
HDD: 320GB Western Digital with IDE interface

I bought my system 6 months ago, from then, I was having problems.
System hanged frequently, vertical coloured lines appered on the screen, system restarted frequently and I could not insert my second RAM into slot B. Even with RAM only in slot A, I had all these problems, after inserting the RAM into slot B, the display will stay blank. First, I send my RAM for replacement and they gave me a fresh new set, the attempt was not successful. Then I replaced my hard disk with a Seagate Barracuda 500GB, again no results. At last I send my motherboard to Intel and they gave me a replacement after one and half month. Even after that I could not insert my RAM into Slot B but Slot A works fine. Please help me

System hanged frequently
I want to discuss about your system hanged frequently. You are using at your CPU.
1. Processor Core 2 Duro E 8400 3.0 G Hz ( Frequency is 1333 M Hz )
2. Memory Zion 1 x 2 GB 800 M Hz ( Frequency is 800 M Hz ) only.

So your processor speed is faster than memory. So Your System is hanged frequently. you should add another Memory Ram.

My advice
You should use DDR2 667 CL5 Ram x 2 pcs.

1. Processor Core 2 Duro E 8400 3.0 G Hz ( Frequency is 1333 M Hz )
2. Memory DDR2 667 CL5 x 2 pcs ( Frequency is 667 M Hz x 2pcs = 1334 M Hz ) nearly same.
If you use like that, your system frequency is nearly same. So you can prevent for system hanged frequently.

Vertical coloured line appered on the screen

I don't know about that. But I thinks, this is not CPU problem. May be it is monitor problem. I also happen now.

I can tell to you, Your Hard disk is Ok. no need to change.

May you happy and peaceful...

My mother board, DG43NB is capable of holding up to 4GB DDR2 800 M Hz RAM and fully support a core 2 Duo/Quad family processors, the answer is not correct

I also thought of ram, but not the hertz.

I have had issues with the BIOS not detecting the timing of my RAM correctly. I researched the proper timings (in my case 5-5-5-15) and the proper volts (2.1V in my case) and told the BIOS this was the settings and have not had any crashes since. CPU-z (google it) can tell you the correct settings in its SPD tab.

This may not be your issue, but certaily something to check.

You are using the motherboard's built in graphics? I've never done this so I don't know whether there could be an issue here. Someone else may.