I'm not sure what your program does, but you can probably achieve better results by using Group Policy.
I'm assuming XP Pro, but this will also work for Win2K, Vista and Win7 as long as they're not 'Home' editions.
With Group Policy you can control access to system settings like control panel, display properties, Internet properties, the run command and regedit. You can also dictate which programs can run, control which drives the user can see, specify what appears on the desktop, etc, etc.
Group Policy can lock down a machine beyond belief. We had a policy file given to us for a super restricted machine where the user had an empty, blue desktop with a red diamond where the Start button normally appears. Clicking the button produced a menu with two options: 'MS Word' and 'Log Off'.
I'm not sure if it works on a single machine, but in a networked environment, Group Policy is reapplied every 15mins, so even if someone does manage to change a setting, it will be changed back by the system very quickly.
Basically, you create a guest account (or even better create a group for Guests and another for Users who will have full access to the machine), use the Group Policy editor (run gpedit.msc) to restrict what the guest account (or group) can do.
Once you've locked down the guests and allowed your account (or group) full control, set a password on your account, allow the guest account to login without a password and you're done.
As a further step you can also set NTFS permissions on drives to prevent guests snooping around your files.
Lastly, you can export Policy settings to a file, so if you get a setup you like, you can easily apply it to another machine.
Here's your start point: http://support.microsoft.com/kb/307882