Well FW and AV will gulp at your memory. It might not showup in the applications (as Interent Explorer does)becasuse you have selected it to startup when your computer starts.
Some of the Processes can be forgotten but i leave them as they are. I have no memory problem in fact 512 is perfect for me.
Also have a look in the taskbar on the right side how many little icons do you have. I have 8 (becasue i have 8 applications open)I have 255MB left...
Sometimes programs install update checkers so they are designed to start up when your computer starts. You can disable a lot things it will give you more RAM but you have to be careful becasue it might not make a program work correctly.
Feel free to click here this will give a huge list of "allot" of programs that are there when you startup your computer like i said you can ignore some of them to free your memory.
If you go to the run command on your start menu and type in "msconfig" without the quotes and click "ok". Now select the starup tab.
Cross reference what you see in the huge list and see if it is safe to disbale them. Just take the tick out of the boxes...
Once finished just restart and you will find you will free up memory.
As for the processes i have forgotten the link so you'll have to wait till someone can come up with that so you can disable some Processes.