Hey RE,
Vista automatically saves restore points, and should keep them for 'years'. If you follow my process by booting off the Vista DVD, select your language/region settings, then on the next screen select repair. You'll get an automatic search for your OS. It should find your main OS installed on C:. It will be highlighted, if you click in the white area to unselect the OS then click next it should drop through to the advanced tools (or if you go through the automatic repair and it completes, there's usually an option to go to the advanced tools, click that). You should see a list of tools, one of which is the command prompt, click this. In the command window type rstrui.exe /offline:c:\Windows, this should start the system restore wizard. There should be a fairly short list of restore points, but there's an option near the bottom of the window to show restore points older than 5 days, click this. You will probably see a much bigger list. Scroll down to find where you installed the Parallels tools (there are 5 or 6 lines of parallels tools), then select the point below this.
In your case though you need to consider what you might be doing here. It sounds like you have been running Vista for some time since installing the Parallels tools. If you use system restore to wind back to before Parallels, you may well 'uninstall' other software too.
I'm pretty sure that re-installing from the DVD works too, but it usually moves the existing Windows folder out of the way, (renames it), then installs a new copy of windows. Doing this will affect a number of installed applications (you'll probably have to re-install applications as well), but it will leave your user data in place.
I suspect you don't really want to do either of these two suggestions!
Best of luck
RTH