Logging on the computer.....

You have to turn on Security Logging option. And check it after you think somebody used your computer. To turn on Security logging you have to do following. Start->Control Panel->aAdministrative Tools->Local Security Policy->Local Policies->Audit Policy. Right click on Audit account logon events and choose properties. And then check success and failture, and do the same with Audit logon events. To check the Security logging, you have to go Start, right click on My computer and go Manage item. Then System Tools->Event Viewer->Security. And you will see what you are looking for. If the intruder has administrative rights on your computer the inruder can delete logs. Then you have use special software which you can find in Internet.

Logging in.

to record specified events

to secure it