The Dell Inspiron 545 uses integrated graphics. This machine dates back to at least 2009. The integrated graphics is going to need memory to function. My guess is that it is most likely using 1gb of memory for that. There are various different CPU and video options, so its impossible for me to know exactly what your machine has.
In any case, Windows is using a nice chunk, the video card is too, and then pieces of memory are reserved for all those devices in the system. So seeing just under 2gb of memory as available on a 6gb system is very possible.