How often does this happen? Is it a continual problem or sporadic?
Try restarting explorer.exe.
Ctrl-Shift-Esc to launch Task Manager
Go to Processes
Find explorer.exe
"End Process"
Go to File -> Run -> explorer
A quicker way may be to restart the computer.
If it is happening often, you need to find out what software is on your system that may be effecting the system.