FS9 Memory Problem

[APX_004]

Hello all,
I came home yesterday from a mini vacation only to launch my FS9 and it comes up with this error:

Your computer has run out of available memory. Flight Simulator will now exit. You may not have enough free space on your hard drive. Run Disk Cleanup to free up space and then try running Flight Simulator again.

May I note that I have 20GB of free hard drive space.
My specs:
AMD Athlon XP-M +2500
1.87GHz
512MB of RAM(Dual-DDR-SDRAM)[2x 256MB's)
40GB HD
64MB ATI Mobility Radeon AGP 4x

*I have tried reinstalling FS9 two times and now I have ran disk cleanup and cleared out all very old System Restore areas. I really don't know whats up *

[APX_004]

Disregard....After the reinstallation(3rd time) it decided it would work. I'm so happy I think I may have just peed in my captains chair.