There are several "memory configurators" online where you just have to enter the model etc of your computer and you will be able to find out what id the max RAM allowed as well as how many slots you have left.
Have you done some of the basics like Disk Clean, Defrag, dump old System Restore points which are memory hogs.
Download and install a small programme CCleaner for a quick removal of a lot of crap.
If you are not familiar with many of the startup entries in msconfig, download another small programme called Startuplite and run it and it will give you immediate good advice about the uneccessary programmes and you can have it shut these down for you. As there seems to be so much on the hard drive, seriously consider getting rid of a lot of the rarely used programmes by doing so in Control Panel, "Add Remove" Many programmes are memory hogs running in the background whether you use them or not.
Regards
carholme