Depending on where you are reading this figure you could be seeing the amount under utilization. For example, if I look under system properties it only show that I have 981,476KB of RAM out of the 1024MB that I know is there.
Then again if you are using something like SIW it will show the total amount and the individual module's amount. If you don't have this, download it and give it a spin. The 64MB is the size of you smallest module, so it could be that it is bad. The SIW (System Information for Windows) will no show the module if it is bad.
e.g my old pc has max total ram of 384mb and max per slot of 128
SIW will show the maximum capacity, it will also show how many expansion slots there are, divide the capacity by the number of slots for the maximum capacity for each slot.
With a motherboard that uses PC100 and PC133 there are a couple of potential problems with RAM. Some motherboards are picky about what RAM they like, my MSI K7 Pro didn't like high density modules. Some motherboards are specific where they want to see the larger modules, like the larger module should go in the first slot.
If you download the SIW it will take a lot of the guess work out, after that you will know if the amount represents what you have installed. If you are still missing that 64MB try removing all but one module and trying each one separately.