A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
OK, so you're opening files by double clicking in windows explorer (you said "when I open Excel", hence my misinterpretation). I have a feeling this is an office 2010 bug, but just to check, try opening a file. Now close the file so you have an "empty" Excel. Maximize Excel. Close Excel.
Any joy? If not, I'm out of ideas. Maybe there's a registry key you can tweak. A quick google threw up this (on this site!)...
Read through it all. There is something you can try with the registry, and some other work arounds, but it does appear to be a bug. Frustrating... I know.
Cheers
Rich