Hi
This has been a problem with Windows Vista and Windows 7 primarily since these operating systems were released. It is to do with the number of "svchost" sessions running in the background. I get it a lot and there is nothing you can do about it other than waiting for the system to return to you. You could check the amount of RAM you have on your system as the more RAM you have tends to reduce the instances of this happening.
Hope this helps.
Good luck.
Tony(OTPM)