I came in to work and found that my desktop has moved around and applications have been closed down. Is there a way to determine when my computer was unlocked?
As mentioned above, you can use Event Viewer to see who logged in. You will probably want to check which event ID to filter on, e.g. 4624 (http://support.microsoft.com/kb/947226) and then find the last entries that don't correspond to your username.
Right-click on Security and select "Filter Current Log"
Instead of enter the Event ID number you want, e.g. 4624