Windows Event Viewer
In Windows 7 if you to Start and type in 'Event Viewer', an application will launch that will allow you to track all login attempts to your PC.
In the Event Viewer menu list, select Windows Log and expand the folder tree. Select security. This will show you a log of events on your PC, and 'Date and Time' field will allow you to easily tell whenever someone has had your PC on.
Note that you may see entries with the Task Category of 'Logon' and with a note of 'An account was successfully logged on' on the General tab in the pane at the bottom, even when the someone has not successfully logged on and entered the correct password to access the system.
If you click on the 'System' icon in the tree, and scroll to find 'Error' level records, you'll be able to find notes reading, "currently configured password due to the following error: Logon failure: unknown user name or bad password." on the general tab. A good way to pinpoint when an unauthorized person was trying to access your PC.