Your MB (which you did not specify) is probably designed to use 1 and 3 for optimum performance when using 2 modules. The low memory message is probably a result of how inefficient Win XP was in allocating and releasing memory as programs are opened and closed. This was often remediated by restarting the PC often. My place of work still uses XP and they recommend weekly restarts for this reason (among others). Even then some software would generate this message even with a fresh restart. Does your graphics system share some memory with the MB? That would reduce the memory available to some software.
not sure it was just a thought trying to explain error message any suggestions on how to get passed the message restarting doesn't help it is locked in start up and I can only go to set up in start menu
Look in your manual to verify the correct memory slots to use when only using 2 sticks. Verify in bios that settings match the ram sticks specs. If overclocking anything then perhaps return to default settings to eliminate that variable. Start in safe mode. Run your scans and verify all ok in safe mode (drivers, etc). Then try regular mode.