Your question depends upon a lot of things, but what you suggest is technically quite possible.
Some things to think about:
I believe you can convert an existing disk to a virtual disk and then run it in a virtual machine, although I have personally never done it. It seems like it would be a common operation though.
The virtual machine is going to present a different set of virtual hardware from what your previous PC was in most cases so there might be some activation issues with Windows to work out.
If your desired application requires any specialized hardware or even real video acceleration the situation can become more complicated. It doesn't sound like this is the case though.
It would probably be a good idea to wait until you had things up and running for awhile on the VM before you trash the original, but I'm sure you thought of that already.
There's an overhead with virtualized cpu but I believe with modern processor extensions its not significant. I know virtualbox allows you to control how many of your processors are presented to the guest OS and you can cap the percentage of the real processor they can use. Changing the ram after the fact is simple as well and the ram will be freed up when the VM is not in use so as long as you have some spare memory capacity I don't think you'll have any problems.