Right click on an empty space in the desktop and choose properties, the display dialog box will open, go to desktop tab, and on right side there is a tab for Position.
Make sure Tile is not selected, use stretch to fill the desktop space.
If this is not something that you did, then check for malware:
Download, install, update and do full scan with these free malware detection programs:
Malwarebytes (MBAM): http://www.malwarebytes.org/products/malwarebytes_free
SUPERAntiSpyware: (SAS): http://www.superantispyware.com/
Remove all found, reboot, and run again until scans run clean, these can be removed afterwards if you so choose.
Also please tell us what your current antivirus is, and some system information:
Click Start, Run and in the box enter:
Click OK, and when the System Summary info appears, click Edit, Select All (Ctrl-A), Copy (Ctrl-C) and then paste (Ctrl-V) the information back here in your next reply.
You can edit out any personal information.