SAPinst has detected a pending installation

I uninstalled the Sneak Preview SAP WAS Developer Edition and then tried to install it again. However, when I start the SAPinst program, the following error message will always appear:

- SAPinst has detected a pending installation.In order to reliably continue this installation, SAPinst has to reboot the machine.

Then I reboot and run SAPinst again. However, the same message appears. I have removed the folder "Programs Files\SAPinst" and start again, the message still appears.

One thing was that I used the "Add or remove program" in Windows XP rather than use the SAPinst to uninstall it. I guess it may cause this problem.

How can I manually delete everthing so that I can re-install the WAS developer edition?