Are u giving ur system to anonymous persons .......just think for a moment ?
There are many techniques available by which u can have no access to ur own account , or hidden account is created without ur notice.
Few of the techniques by which the password of ur account are changed without knowing the old password (Note that the change of password without knowing the old password is not allowed under user accounts of control panel) are:
Using GUI method
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-->Search for LUSRMGR.MSC
-->Click on the users under Local users and groups(Local)
-->Right click on any of the accounts (Administrator/Guest/any account)
-->choose set password and then proceed and then it allows for the setting of password
Using Command prompt
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-->Right click cmd and Run as administrator
-->type net user username *
-->it would ask u for setting of password
so, i suggest that you not to give ur systems to anonymous persons and also to create a password reset disk of ur system(It can help u to reset the changed password,no doubt when u have created the password reset disk)