Over the course of a day, if you start and quit your word processor a dozen or so times along with other applications, losing a bit of memory each time, your computer eventually feels the pinch, runs out of memory and sometimes crashes or locks up without warning. If you are using an older computer with minimum internal memory, it's not too hard to push the memory limit. The reason it usually doesn't happen in the morning is because you haven't turned your applications on and off many times and therefore the loss of memory has not accumulated to the point where it interferes with program operation. There are several cures for the problem. First, you could install more memory.
More memory is always a good thing and has never been cheaper.
With extra memory, the little chunks you lose during the day may not so readily challenge your computer's memory limits. A second way to deal with the problem is to reboot your computer during the lunch break. Rebooting restores any lost memory. Finally, be aware that Windows is a multi-tasking environment. That means that you can run several programs simultaneously.