How to Setup Mirror Server for critical servers to have redundant boxes?

Basically to sum it up we have a few older servers running some business critical applications. I say older because they have only one power supply and a very simple RAID 1 configuration. I'm looking into ways to have a backup ready in case it fails.

I'd like to mirror our server onto one of our spare desktops in the case of failure, so we can plug in the desktop in the meantime (obviously slower), while we do a server rebuild. Is there any windows based software that is cheap or effective at mirroring the contents of a server exactly? I would never expect the performance to rival it, but a slow server is better than no server.
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Install ESXi on a server and do a P2V. VMware Converter can create a new virtual machine by importing an existing machine. Should the physical machine crash, you can start the virtual machine to restore the server role.
You can build on this of course, but the cost will increase:
Clusters with the active node physical and the passive node as a VM on ESX
Or make you're critical servers all virtual on an ESX cluster. If an ESX host should crash, the VM is restarted on an other ESX host.

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danielevans83Author Commented:
VMWare is nice, but we do not actually own it so that would add a monstrous bill on top of this project. We were looking into doing this not using virtual machines, but spare desktops. I'd have to manually wire the spare in obviously, but right now I'm just looking for the software capabilities to actually mirror the entire server onto another computer.

Or is what you're saying that you can only do this with VMWare?  Is this because VMware creates an exact duplicate including hardware specs?
ESXi is free. There is no cost attached to it. It has a lot of limitations and all the nice features are disabled (vmotion, HA, DRS, ...), but you don't need those for your "standby" servers. So you only need the hardware.
Look here for the details:
danielevans83Author Commented:
We actually own a copy of MS Virtual PC or Virtual Server or whatever it's called. ver 2005.

Would that also work or should I just go with ESXi?
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