Mel;
If the computer is truly powered off, this is a BIOS issue and not a
Windows issue.
Go to BIOS and look for something like "Wake on ring", normally under
Power tab.
Disable it.
If you are unsure how, check your manual or contact the
computer/motherboard manufacturer.
Yup. Just to clarify, getting to the bios can be different for everyone,
usually you have to press del as the computer is starting up, it may say
something like 'press delete for settings'. Be careful changing anything
that you are unsure about, and always rember the original value incase
something goes wrong. Messing up the bios can stuff up your computer...