Cannot install the vCenter agent service Not enough space to install agent ESXi 4.x

I have been building a very basic home lab today, and came across this error when trying to add the host into vCenter.

“Cannot install the vCenter agent service. Not enough space to install agent.”

I began investigating the error, which to me sounded like a disk space error.  The host is running ESXi 4.1 off a USB key.  I checked the disk space and all was fine and acceptable.

What I discovered was that a VM that was running on the host had eaten a huge chunk of memory, and there was only 128MB free (this host only has 2GB RAM, not ideal I know).  I stopped a few of the VMs which free’d up some memory, and then tried to add the host to vCenter again.  Everything worked well.