Our administrators have disabled the screensaver and automatic lock, so i could not use that. To prevent me from leaving my pc unattended, i did this: How to lock Windows XP when Idle
%windir%\system32\rundll32.exe user32.dll, LockWorkStation
That’s it, It should auto lock your PC after it reaches the set idle time.
If you can't use tasks, try http://www.dmvsoft.com/lock-your-pc.htm