Why does Windows show only 2.5 GB memory when BIOS shows 4 GB? [duplicate]

Solution 1:

My Guess is you've got a 32bit OS, so your limit is down to 3.5GB then you've probably got the onboard video cranked up to the max utilizing the missing 1GB of RAM. You should be able to turn the video down or just pickup an actual video card and pop it in to regain the other 1gb or RAM.

Solution 2:

Windows XP has a 4GB physical memory limit, but things like graphics cards and other devices can affect this. Userland memory is also limited to 2GB.

Take a look at this: http://msdn.microsoft.com/en-us/library/aa366778%28VS.85%29.aspx

And this: http://www.brianmadden.com/blogs/brianmadden/archive/2004/02/19/the-4gb-windows-memory-limit-what-does-it-really-mean.aspx