Computer CPU(intel pentium D 925 processor ) error

emachines computers are well known to have el-cheapo power supplies that tend to malfunction more often than average, and when the power supply fails completely, they are a lot more likely than average to damage something else, often the mboard.
Unplug the cord to the computer, or otherwise switch off the AC power to it, open up the computer case, and find the label on the power supply - if the brand is BESTEC I advise you, if you find ANY indication the power supply might be in the process of failing, DO NOT trying booting the computer anymore - if the PS fails completely there is a strong likelyhood it will trash your mboard!!!!

Failing power supplies are common and can cause your symptoms.
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" have run Memory test and memtest, the bootable memtest came up with no errors in ram."

I that case, there's probably nothing wrong with the ram.

" the memory test that runs in windows came up with many errors, i have scanned all sticks of ram sepertately and still found errors in every stick."

Your problems may be because of a poor connection of your hard drive, or the hard drive is starting to fail.

It is common to un-intentionally damage IDE data cables, especially while removing them - the 80 wire ones are more likely to be damaged. What usually happens is the cable is ripped at either edge and the wires there are either damaged or severed, often right at a connector or under it's cable clamp there, where it's hard to see - if a wire is severed but it's ends are touching, the connection is intermittant, rather than being reliable.
Another common thing is for the data cable to be separated from the connector contacts a bit after you have removed a cable - there should be no gap between the data cable and the connector - if there is press the cable against the connector to eliminate the gap.
80 wire data cables are also easily damaged at either edge if the cable is sharply creased at a fold in the cable.

Try another data cable if in doubt.

Check your SATA data cables. The connector on each end should "latch" into the socket on the drive and on the mboard, or on the drive controller card - it should not move when you merely brush your hand against it near the socket - if it does, mere vibration can cause a poor connection of it - use another SATA data cable that does "latch", or tape the connector in place.
(There is a slight projection or bump on one side of the outside of the connector that "latches" it into the socket - it's easily broken off or damaged)

The same thing applies for the SATA power connection.

 If the hard drive is starting to fail you could certainly have errors in Windows that you don't have when you boot from a floppy.

Check your hard drive with the manufacturer's diagnostics.

If you don't have a floppy drive, you can get a CD image diagnostic utility from most hard drive manufacturer's web sites, but obviously you would need to make a burned CD, preferably a CD-R for best compatibilty, on another computer if you need to.

If the hard drive itself tests okay, any data problems found can be fixed, one way or another.
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"I also read online that memory errors can appear but they are actually motherboard errors."

That's extremely rare, and if that were the case, it's very likely the ram would fail the memory test booted from a floppy as well as in Windows.
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Contrary to popular belief, it is extremely rare for ram to go BAD when it was working fine previously.
The ram you had in it in the first place is probably fine.
However....
A common thing that can happen with ram, even ram that worked fine previously, is the ram has, or has developed, a poor connection in it's slot(s).
try cleaning the contacts on the ram modules, and making sure the modules are properly seated:
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Apparently the chipset on this mboard does not support using DDR2 800mhz (PC6400) ram.
The DDR2 800mhz spec itself is backward compatible, but that's not the only thing that determines whether the ram is compatible, and the problem is very few places list DDR2 800mhz ram for this mboard.