"out of memory" when there is plenty of free physical memory

[palomino]

host os: windows 2000(2GB ram,about 1.5GB free)

guest os: windows xp(512MB),ubuntu(384MB),slackware(128MB)

I just ran one VM at the same time.but only slackware worked.other 2 VMs got error like this:

Failed to lock 536870912(402653184 for ubuntu) bytes of host memory (out of memory). VBox status code: -8 (VERR_NO_MEMORY).

Result Code: E_FAIL (0x80004005) Component: Console Interface: IConsole {1dea5c4b-0753-4193-b909-22330f64ec45}