32 bit Windows has a limit to the amount of application memory it can address regardless of the amount of internal RAM you have installed. Your memory can become fragmented and this means that you may receive an 'Out of memory' error during a backup even though Windows task manager shows memory available. To overcome this Windows can be set to allocate more memory to compatible applications that are configured for IMAGE_FILE_LARGE_ADDRESS_AWARE. Macrium Reflect is configured for this.
Enabling Large Address Support
The steps you need to take to enable large address support depend on your operating system.
On Windows 10, 8, 7, Vista, Server 2008 R2 and Server 2008
To enable PAE and 3GB user mode memory on Windows 7 and Vista download and execute 'EnablePAE.exe' from here: http://updates.macrium.com/reflect/utilities/enablepae.exe
On Windows XP and Server 2003
Modify the 'c:\boot.ini' file, add the '/PAE' and /3GB switches and reboot.
Example boot.ini file with /PAE and /3GB:
[boot loader] timeout=30 default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS [operating systems] multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP" /FASTDETECT /PAE /3GB
More information on XP boot.ini switches can be found here: http://support.microsoft.com/kb/833721