Did your desktop change right in front of you (one moment YOUR desktop, then some random change to something else?)
When you say that it changed - did it change to the original default background?
Another indication could be that your Dock does not look the same.
Have you noticed anything else that might mean that your system settings are reset - things like screen resolution, window settings, or even the menubar
Have you noticed other changes recently, such as much slower perfomance, or sudden freezes, when you have to force a shutdown?
Any of that can be a symptom that you may have a hard drive that is beginning to fail - or perhaps you have a permissions issue...
(Even in the unlikely event that your computer is hijacked, a malware scan with Sophos, etc, won't report anything about that)
Boot to your recovery partition, and run Disk Utility/Repair Disk.
Then, run Repair Disk Permissions.