that is not physical memory but virtual memory, is your hard disk partitioned into two? that is you have a drive C and a drive D but only one physical hard drive?
if your drive C's (where the default virtual memory is allocated) free disk space is low then you might be low in virtual memory as windows could not allocate more for it.
you can however allocate some disk space from your drive D for your virtual memory.
go to [Control Panel][System] Advance tab
click the 'Settings' button under"Performance"
go to the Advance tab under Performace Options
under the Virtual memory click the "Change" button
Choose D: (it should show you the Space available for allocation)
tick the option button adjacent Custom Size:
for the Initial size(MB) allocate around 150MB
and for the Maximum size allocate around 450MB (or whatever size you deemed needed...
If you are not sure then you can just let the system allocate for you by ticking the option button opposite "System managed size
click the Set button and click OK and OK again until you have closed the system dialog box closes...