Difficult to tell without a little more information.
Have you added any new software in the last few days? While unlikely it may be woth checking for a virus or other malware.
If you press Ctrl, Alt and Delete together you will get a list of programs that are running. Perhaps one of these may be slow (or failing) to close itself down, which might require a reinstallation of that program.
It could also be hardware related (e.g. hard drive) but I would expect you to have other problems while using the system.