DOS
When creating a virtual machine with MS DOS (or PC Dos) then you'll notice that there are a few issues.
The first major issue being that the VM will run one of your cores with a high CPU. The reason being that MS DOS did not idle your CPU when it wasn't doing nothing, so instead of having a VM that uses no CPU when not doing anything you have that 1 core going nuts.
The classical way to resolve that is by installing DosIdle.
As it doesn't appear to be available much, here's a copy.