Basically the problem is that the virtual machine (XP mode) is created under the first user account profile so this will not be able to be used by other users. My guess is that the solution is not trivial. I reviewed the following link and it's not clear whether the users there had any real success. At least there is nothing there that doesn't look contorted in some way.
http://social.technet.microsoft.com/Forums/en-US/7036a4da-5764-4a95-a52a-a2e93f39e27a/windows-xp-mode-1-vm-for-multiple-users?forum=w7itprovirt
You might be better off trying a different virtualization tool, (provided you have XP licenses), such as Oracle Virtual Box or VMware Workstation. These tools give you better control over where virtual machine files are saved (you can save in a public location), but the caveat is each user will need to know how to use those tools.
I don't use Virtual Box often (I use WMware a lot, and used to use MS Virtual PC and Virtual Server a lot), but I believe it works directly with *.vhd files so you might be able to use your Windows XP mode vhd that way.
http://www.sysprobs.com/how-to-open-run-microsoft-vhd-files-on-virtualbox-pre-installed-vhd-images
Sorry about the links, but there is a lot of information (including screen shots) in there. I'm trying to point you in a direction that gives you some options.