Hello Christian,
It looks like the code you are using is identical to that given in the VIX API Reference:
http://pubs.vmware.com/vix-api/ReferenceGuide/lang/com/functions/PowerOn.htmlI don't think TestComplete affects the way the script works - I think there is a problem in the script.
To check whether the script itself works, put it to a VBS file (for example, Test.vbs) and run the script by double-clicking the file in Windows Explorer.
If your script works when put in a VBS file, but does not work when executed from TestComplete (without any modifications), let me know.