How to login to SYSTEM user in Windows 7?

Use PsExec from Microsoft with the -s and -i options.

To start a command prompt running in the local system account.

Add the file containing PsExec to your PATH environment variable.

Start an elevated command prompt by right-clicking cmd.exe and clicking 'Run as Administrator'

Enter the command: psexec -sid cmd.exe

See the example 42 minutes 20 seconds into http://channel9.msdn.com/Events/TechEd/NorthAmerica/2012/SIA311

If this isn't what you are trying to do, I have misunderstood your question.

Yes it is but it is hard to explain. Well technically your logged into the system account when your not logged into your account. Change Sethc to any other name then copy cmd.exe and name the copy sethc then on login press shift 5 times. it opens CMD.EXE. then type in explorer.exe then you are in the system account!