I have been using Remote Desktop Client from Xtralogic, Inc. It's on the order of $20, but works great. I've been using it on my phone for about a year and now on the Xoom.
There was a problem on day 1 with the Xoom and Windows 7. However, a quick email with Xtralogic and they had a new version released on Friday last week to fix the problem.
Here's what I like...
It uses remote desktop protocol in Windows, so there's no third party software to install on the PC.
Full screen is great on the Xoom. 1280x800 is the same resolution as my laptop, so no degradation in viewing the screen (except a bit of pixels lost on the bottom for the perpetual Android "buttons").
Two modes for the mouse - either touch screen (painful on the phone, but probably OK on the Xoom) and track pad (or something similar) type where you turn your display into a mouse pad and scroll around, tap to click, etc.
I read some reviews a year ago when I made my decision and haven't looked back. $20 well spent.