This article could just as well be called “All security bets are off with physical access to a system”. How-to Geek has a great writeup for those who find themselves needing to get into a Windows 7 system and either have lost the password or don’t know it.
While the article is a helpful how-to, it illustrates in amazing clarity how, no matter how complex your password is, once someone has physical access to your system, it’s pretty much as if there was no password at all.
The process is amazingly simple but it does involve overwriting files, so remember to always make backups. Their writeup is amazingly detailed, so I’m not going to post it here again. Hit the link for the full article.