How can I access the System Account in Windows 7?
Try psexec from Sysinternals as mentioned in this Stack Overflow answer:
[...] I came across this article which demonstrates the use of PSTools from SysInternals (which was acquired by Microsoft in July, 2006.) I launched the command line via the following and suddenly I was running under the Local Admin Account like magic:
psexec -i -s cmd.exePSTools worked great. It's a lightweight, well-documented set of tools which, in my opinion, provide an appropriate solution to my problem.