There are a few reasons this might happen
1) the system is bogged down and the response is VERY slow
fix=clean startup
2) the Windows crawl is affecting the desktop icons
fix=reduce index and rebuild
3) there might be System File corruption
fix=SFC
4) Malware
fix=Scan/clean, scan/clean, scan/clean by multiple malware pgms .... until healthy scan reported
It sounds bad, but only if Malware is the cause should you be concerned, everything else is relatively easy to check and fix.
I offered my suggested fixes below each list number in case I get distracted - another member can see the direction I was taking.
Clean Startup, see:
http://www.sevenforums.com/tutorial...ation-conflicts-performing-clean-startup.html
If the issue is intermittent, it might take a while to determine if that solved it. You know better than anyone on this side of the monitor how frequently this occurs (it might be helpful if you stated that = every hour, every day, every week, once a month....)
There isn't any real need to do any of the other things if a Clean Startup solved it - right?