Cannot allocate all memory

Hello,

I am having this problem just this morning:

I installed ESXi 5.1 on a host machine which has 4GB of memory. The host console screen indicated 4GiB of memory after installation

But when I use vSphere client to deploy VMs on it, it shows Total capacity of the host as < 2GB of memory. As a result, I can't power on a 3GB RAM VM

Please, somebody help?

Thanks a lot

PS: I have attached some screenshots for your convenience