I’ve checked just about everything, Hard drive OK, PC turns on does not start system, Monitor OK, I think is the mother board or the memory card.
Specifics will go a long way to helping you. You need to tyupe out what is exactly happerning and what isnt. Read Oran's reply and answer accordingly, then we will be able to answer and offer a solution.
Motherboard is good, for sure, or else it wouldn't even turn on. RAM maybe partially broken, happened to me once and the OS wouldn't start becaue of that. Go to the BIOS and confirm if the RAM reported in there is the amount you really have.
Reinstall the OS. It's the only other option I can figure.
If the the PC turns on but then nothing how have you checked everything! If motherboard is dead you would have nothing at all. If you get a single beep that means motherboard seems ok check elsewhere, do you get any other beeps, try reinstalling the OS.
There are two things you haven't mentioned:
1. When you switch on the computer, does _anything_ switch on (fans, any writing on the screen, beep, hard disc, lights on front)?
2. Following on from the above question, have you tried a different power supply? In my experience power supplies tend to fail more often than motherboards, and the failure can be gradual, so that the system may try to boot up and fail, or not switch on at all.
Finally, I assume that by "memory card" you mean the memory module (RAM) rather than something like an SD card. Normally, if there is no memory in the computer (or if it is incompatible or faulty), the computer will beep a number of times, so if you are getting such beeps, consult your motherboard's manual to find out what's wrong. If you're getting nothing at all, you need to assume it's the motherboard or processor. It's virtually impossible to test these other than by substitution (that is, trying another processor or motherboard). Unfortunately, this can be expensive and it is often more cost effective to replace the motherboard, CPU and RAM with a newer model, but if you can test your components, by swapping them with others, which are known to work, then do so.
well do you have actual windows OS inside the hard drive, or it is deleted? can you boot on live cd like hirens boot cd?
Do you hear any beep and how many when you start pc?