I only had this happen once and it was because I had the incorrect stick installed. Back when I was young and dumb, I thought they were interchangeable.

Thanks...  

Your PC should have an upgrade info you can pull up to tell you exactly what you need..or you van use Everest Home Edition to give you a complete list of all your PC software and hardware.

I wouldn't think this would be a problem for you? Did you check to see you were putting in the right stick?

I had tried to put an old dimm memory on a new PC. But then, I'm an idiot.

Everywhere I did a memory scan it says I have 4gb's but it doesn't register on system properties or on a simple dx multi gb meter I have. When I enter setup (F2) on keyboard under memory info it says 4.0 GB's. I guess I have it, it just won't register on system properties or I think task manager either...  

What kind of computer do you have? I don't know what you have, but in your all programs. at least, you should have a help section just for the maker? that you can find some or all of the info you need.

Do you know what memory stick you need?

Is it set up to handle 4GB of memory?

That is what happened on mine...not that this is the same problem you are having...but it was due to the wrong kind of memory and then too much (Mine could not handle more than 2GB's)..it would show on the bios but not when the PC was booted.

because in win xp (if that's what your running) will only read the max of 3 GB's cause XP does not have a good PAE (physical address exchange) so you could have 20 gb's of memory in and it will only read the max of 3 gb's

Is there a way around this that you can be sure your memory is installed right? or software that will tell you?

All the memory is right it just won't register it, like whatwhere says. But I don't see any real improvement so I'll probably send them back and just reinstall the old ones which are 512 each (2 of them). So instead of 4 gb's I'll have 3 again.. Thanks everyone.. By the way it's a Dell 4700..  .

What I am seeing is that XP only supports 2GB and Vista 3GB (though Vista may show 3.5) and they don't really utilize the rest. However, some plugins and graphic apps want 2gb free and 'they' may utilize it, but it seems to be only virtual memory that is being mentioned. Everything clearly refers to 32bit systems. Would 64bit read all 4GB's?

edit...here is a thread I found on it...

WWW Link

Thanks..  

the only way your going to know if all 4 GB's are installed is in the bios but
the OS won't detect all 4 GB's although they might be some programs out there that might be able to tell XP the correct amount

This link applies to the Dell4700 and has the info on type, how much and what you need to run 4GB's.

WWW Link

You may only need to add the /PAE switch to your boot.ini file
(Some fun MS reading on the topic:)
WWW Link
WWW Link
WWW Link
WWW Link

If that doesn't help, flashing your bios to the latest version may do it for you.
*Careful, or you'll turn your system board into a nice paperweight.

Or you can wait for yrag to stop by and set us all straight.