When I connect remote desktop via the lan, the remote PC is switch off.But why?

Is it switched off completely? Most remote admin appz had a "Disable Screen of Remote Computer" option. Just check you settings again.
Also check that your remote software is up-to-date. I had a issue similar once, I just got a blue screen every time I connected. Turned out the software I used was old. Got a new version and presto.
If all els fails, try other remote admin appz. Www.LogMeIn.com is very nice.


Hope this helps
Grant Hills
South Africa