In most cases, the ability to support various levels of encryption is actually a function of the wireless hardware being used. That means that there are wireless adapters that can support WEP just fine, but cannot support WPA or WPA2 no matter what you do. There's no way to just "turn it on in Windows" because it's not Windows that's the limiting factor, it's the hardware.
The easiest solution is to simply get another wireless adapter. They're not expensive, and can be had in a variety of form factors. In particular, a plug-in USB wireless adapter seems ideal for the scenario you're outlining. You need to disable the old internal adapter and you should be good to go.
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