It is easy to return the amount of free memory in windows, using the GlobalMemoryStatus API call. Insert the following into a module's declarations section:

Public Type MEMORYSTATUS 
dwLength As Long 
dwMemoryLoad As Long 
dwTotalPhys As Long 
dwAvailPhys As Long 
dwTotalPageFile As Long 
dwAvailPageFile As Long 
dwTotalVirtual As Long 
dwAvailVirtual As Long
End Type

Public Declare Sub GlobalMemoryStatus _
Lib "kernel32" (lpBuffer As MEMORYSTATUS)

Now, add this code to get the values:

Dim MS As MEMORYSTATUS 
MS.dwLength = Len(MS) 
GlobalMemoryStatus MS

' MS.dwMemoryLoad contains percentage memory used
' MS.dwTotalPhys contains total amount of physical memory in bytes
' MS.dwAvailPhys contains available physical memory
' MS.dwTotalPageFile contains total amount of memory in the page file
' MS.dwAvailPageFile contains available amount of memory in the page file
' MS.dwTotalVirtual contains total amount of virtual memory
' MS.dwAvailVirtual contains available virtual memory

You could use this in about boxes or making a memory monitoring system.