Is there a command to find out the available memory in Windows?

It takes some time (around 10 seconds for me) but the following command will do it:

systeminfo |find "Available Physical Memory"

This will do it without taking 10 secs. Try this:

For Total Physical Memory

wmic ComputerSystem get TotalPhysicalMemory

For Available Physical Memory:

wmic OS get FreePhysicalMemory

Well if you are on Windows 7, you can use this at the powershell prompt:

(Get-WMIObject Win32_PhysicalMemory |  Measure-Object Capacity -Sum).sum

Or if you want a nice pretty how many gigs is it:

(Get-WMIObject Win32_PhysicalMemory |  Measure-Object Capacity -Sum).sum/1GB

Or if you are on an older version of windows (or W7 for that matter) at the command prompt:

wmic memorychip get capacity