How can I get an automatic alert by SMS or mail when my Windows office computer is logged onto/off of?
My suggestion would be to use a command line tool like blat or bmail. You then create a batch script (*.bat) for sending the mails. This could be triggered by just putting it in your startup folder or for a nicer way, see this question: How to schedule a task to run when shutting down windows
if you want to add a Startup/Shutdown script, you need to be looking for Windows Settings under Computer Configuration. If you want to add a Logon/Logoff script, you need to be looking for Windows Settings under User Configuration.
Run gpedit.msc (Local Policies) Computer Configuration -> Windows Settings -> Scripts -> Startup or Shutdown -> Properties -> Add
Run gpedit.msc (Local Policies) User Configuration -> Windows Settings -> Scripts -> Logon or Logoff -> Properties -> Add