Without warning, once every couple weeks, I will notice that my Excel will
stop behaving normally: when you try to enter a value into a cell, you
can't. Excel will highlight the cell instead. If you try to move from that
cell, you end up highlighting whole sections of the spreadsheet. The only
way I have found to stop this behavior is to reboot the computer. I am
currently using Excel 2007, although I had noticed this phenomenon occurring
ocasionally with my previous edition of Excel 2003. My question is: Is this
a glitch in Excel, a virus, or a deficiency in my PC? Any fixes available?

PhillyRon