You need the dev tools installed in order to side load, it installs the device drivers & such (because the dependency of Microsoft.Smartdevice.Connectivity.dll assembly). You also need Microsoft .NET Framework 4. You can then set up WP-Deploy & side load. As long as you are on the latest service pack you should be able to run the dev tool & install all necessary drivers.
I also do recall that somewhere on XDA, a dev had created an app that will allow you to sideload but If i recall it was only for homebrew, not marketplace apps with protection. This app, when you install it, installs the necessary files you'd also get from the wp dev tools package. I'm having trouble finding the thread on XDA though.
Finally, the only other option I know of, since you have an HTC device, is maybe install DFT's mango Rom that has built in XAP installer..where you can download a xap with internet explorer, straight on the device & deploy it. Take a look at this thread at XDA.
