You mentioned "double-clicking" on the exe. Right-click on the same EXE and choose Run as Administrator. This is how you install the software such that the installer has Admin rights.
With regards to "being unprotected" and "not recommended". What it means is that after April 8th, Microsoft will not be providing security updates for Windows XP and they may remove the software in question from their website. They are making such statements to avoid legal issues should a computer become infected or an organization be taken offline because of a proliferating virus due to an unprotected Windows XP.
Here's my advice. You can install and use the Virtual PC but if you ever want to browse the web, be sure that you ONLY do that through Windows 7 so that you have all security patches. I only recommend using the XP Virtual PC for purposes of downloading your Wizz.it config to the remote.
Note that you can edit the config in Windows 7, it is likely only the download for which you will require the Virtual PC and the USB driver. ll this may be a moot point as the RC5400 I don't think has a dedicated driver (like ProntoNG.sys for the TSU7000). That being said, maybe the right-click of the exe and 'Run As Administrator' may be all you need.
If you'd like to discuss offline, ping me @ my yahoo mail addy and maybe I can walk you through it. Will do my best but have not installed the stuff on Win7 (Yet).
