Which is which?

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I just wanna share this experience of mine. A neighbor ask me to repair her computer. It keeps on restarting i think before it reaches windows logo. I immediately reformat it. Windows XP SP3. I successfully installed the OS though there are times that I need to replace the installation CD as the CD Drive can not read some files. I test it with a few restarts and everything seems ok so I delivered the computer. I setup the computer and turns it on. It wont open. It says I need to put a proper boot disk. In short it can not detect the HDD as I checked the CMOS as well. Made a few re-seating and restarts and it goes in. After a day she calls me back. Now with a blue screen. Blue Screen says the system is not APIC compliant and must check manufacturer whatsoever. I check the internet and the only option left to me is to flash the BIOS. I will not. I dont think its the RAM but still i check it with memory diagnostic apps. So funny It cant even run DOS. it hangs on the process. I go back to basic. CHeck RAM and found out it has 2 stick. DDR266-256MB and DDR400-512MB. I check the internet and it says it is fine to combine the two. Still I remove the 512 since it is the newer. Whoalahh! everything is now OK. Installed OS and made a few restarts. Thinking that maybe the 512 is not bad, I remove the 256 and try if it will goes in. It did! How? and what about the error saying about the APIC incompatibility which has no connection with RAM. What is the culprit?