Vista - VPC 2007 SP1 mouse cursor is insane?

Some clients of mine recently updated their computers and asked me to set them up for them. They were migrating from 32-bit XP Pro SP3 to Vista Business x64, and many of the programs they use daily as part of their business had issues with the transition. Eventually, though, I got Vista fully updated and 90% of their programs working. There is one program, however, that is fully incompatible with Vista of any type. For this, I installed VPC 2007 SP1 and used some old XP discs that I had lying around. For a few days, this setup worked brilliantly.

Now, however... On one computer, the setup works fine. The whole thing goes just as it's supposed to. On the other computer, once the VPC captures the mouse, the cursor gets trapped in the lower right corner of the VPC window. Moving the mouse exceptionally slowly allows the user to have a cursor, but the second that it gets jerked, or the user gets overconfident and moves the mouse just a tiny bit faster, the cursor jumps back into the lower right corner. MouseKeys works, but this option is unacceptable to my clients.

I have:
-Restored the virtual machine and .vhd files from two separate backups made when everything was working fine
-Checked all of VPC's settings on the computer that works versus the computer that doesn't.
-Tried a different mouse
-Tried it with Hardware Virtualization enabled (it was disabled when everything worked, and continues to be disabled on the machine that is still working)
-Messed with Mouse settings in the VM's Control Panel, including an attempt to update drivers (no luck)
-Uninstalled/reinstalled VPC 2007 SP1. I used the same VM/.vhd files, though.

I have not:
-Run any updates on either VM. Both XP installs are running vanilla XP (Version 2002, say the boxes the discs came in). I figure this is safe, as the VMs are only going to be used for one purpose, ever. I mean, I've only installed two programs on them: the one my clients need and Adobe Reader so they can read/print the PDFs that the first one downloads (from a secure server).
-Installed the Virtual Machine Additions. Like I said, there's only one reason for these VMs to exist. Speed, etc., is not required.
-Reinstalled XP. I really don't want to unless I have to.
-Tried to run a repair install of XP. Same excuse as above.

These VMs are running on the same hardware:
-HP d5200t
-Wireless optical mouse that shipped with the tower (but a wired USB optical mouse didn't make any difference and the wireless keyboard that came with the tower works fine).
-The tower is running Vista Business x64.
-The Virtual Machine is running a vanilla, completely un-updated 32-bit XP Professional install.
-Each VM has only two programs installed on it other than the OS; both are trusted.
-No settings were altered on either machine until something went wrong.
-Both machines even have the latest VPC update from Windows Update.

Because this problem does not occur on the host machine, I believe that something is wrong with either VPC or the XP install. But two things confuse me: I restored a back-up of the VM made prior to the problem occurring, and the problem still occurred; and I uninstalled/reinstalled VPC 2007 sp1 and the problem still occurred.

In short, I'm stumped. Anybody out there got anything on this?