Never heard of A-Data tbh.
What info does CPU-Z
give on the DIMMS?
Try to boot with one stick at a time to check for bad modules.
Also, ctrl+alt+del and check out what background processes are running. If this is a pre-built PC then you may have a lot of bloatware - unnecessary software taking up valuable Memory.
As a final though, check your page file settings and make sure it is active.