With a VM application like Virtual PC (or VM Workstation or VirtualBox) how do you keep her from getting into the main PC when she has to log into it to get to her VM?

Also, I'd guess she doesn't play any games (etc), as the 3D performance tends to be abysmal on VMs, but as long as she doesn't need it, it's an irrelevance. (* there are some exceptions, where you can pass the GPU entirely to the VM, but they are currently a bear to get set up.)

The alternatives would be a hypervisor where there's no outer OS to log into (or a very thin one she can't get into trouble with) like ESX/Xen/Hyper-V. The free edition of Hyper-V is likely your best choice there (eta: and by "best" I mean easiest to get set up if you're familiar with Windows.)

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As an alternative, what about a second cheapie USFF desktop with a KVM switch? Something like an Intel NUC or Mac Mini is even smaller than a laptop, and would share the same screen.

Also, given her propensity towards picking up malware, any chance of getting her onto Linux or the MacOS? Neither's inherently that much more secure than Windows, but both are the targets of less malware at this point.

Lastly, if still on Windows, any chance of keeping her on a non-administrative account?