You say you have multiple applications running then try to open another. How many apps, and what type?
What is the exact message, with any error code?
What prevents you from closing down some of the running applications then trying the one you want to open again?
You mention 3GB RAM and Physical Address Extension, but isn't PAE enabled to allow 32-bit Windows systems to use more than 4 GB of physical memory. I know there are other uses, but that's the primary reason isn't it? If so, enabling PAE for 3GB of memory seems a contradiction.
Also I understand, (but I am not sure), that 64-bit versions of Windows do not support PAE however I have to confess that I get confused with 32/64 bit processors and 32/64-bit Windows. So take that with a pinch of salt.
When I have multiple apps open in Windows XP and try to open another, I get a message that the app can't open another window.
AMD Athlon 64 X2 Dual
Core Processor 4400+
2.21 GHz, 3 GB Ram
Physical Address Extension
MS Windows XP Pro
I have just recently reinstalled, but I have several utility programs running.