Out of memory error

Hello everyonel!
I am a fresh member of your forum and I found it searching desperately for some experts who would help me with the problem I face!

Some time ago I bought fsx, as I am one of those who love to fly to the heavens! Until recently, the game was playing normally ... I made many hours of flights without experiencing any problems.. Before few days, maybe weeks, happened the following to me:

As I begin my flight normaly, after some time (which can range from minutes up to hours!) I get this message: "Your computer has run out of available memory. Please restart Flight Simulator and select different graphics, scenery or traffic settings " pressing OK closes the game immediately, having, as a result, lost many hours with no point...!

If I do a flashback just before this problem beggin, I installed to the game two add ons: 1-World of AI trffic and 2-Ultimate terrain x Europe ... Also, I confess, that I had mattered the game settings and graphics, maximizing them, but when this message appeared I minimized as before... I tried to solve this problem in many ways looking on the internet, but nothing could help me... I Uninstalled the game,and I re-installed it! After that, I just installed the FSpassenger and ultimate terrain x europe with Default graphics, but still keep getting this message ... I, also, Uninstalled and Ultimate ter x europe ... But the same problem!
What can I do to make my game as before? What has happened?
Searching for solutions to the Internet, the only ones I could not do, because I do not understand how, is to look for corrupt files and for an addition to the boot.ini... I really have no idea how to do these things!!

Let me add, that my system is:
Windows 7 Home Premium
Service Pack 1
512GB memory
Ram 2GB
NVidia GeForce 8500
Intel Core 2 Duo CPU

PS: I apologise if there is something misunderstandable in my message, but I've tried to use my best english which I know... I'm from Greece, so that's not my native language... I ask for your understanding!
If you can't understand something of what I'm writting, let me know!

I would be grateful for everyone, who can help me...I wanna fly to the sky again, as before..!
Thanks in advance!