One of the weird and annoying things about Excel is that it shows you an item on the Windows taskbar for each document that you have open, but it doesn't let them operate as separate windows.  If you're using dual or multiple monitors you'll notice that you can't place one document on each monitor to see them side by side because they share the same window.  The following steps will allow you to open Excel documents side by side, but they involve some tweaks to the file system settings in Windows.  If you don't fully understand the changes below I would recommend that you not make them as they will be a challenge to fix on your own and could mess up your system.  To get Excel to open documents in separate windows so you can place them independently you can do the following:

Excel 2007:

• Click the Office Orb in the upper left corner to bring up the menu.
• Select Excel Options from the bottom right corner of the menu.
• Click the Advanced tab.
• Check the "Ignore other applications that use Dynamic Data Exchange" box and save.

Excel Versions Prior to 2007

• Tools|options|General tab|Ignore other applications (check it).

Then in Windows Explorer

1. Go to Tools -> Folder Options.
2. Select the File Types tab.
3. Select the XLS file type and click "Advanced".
4. Uncheck the "Browse in same window" checkbox if it is checked.
5. Select the "Open" Action in the Actions listbox and click "Edit".
6. In the "Application used to perform action" textbox add "%1" to the end of the line and be sure to include the quotes around the %1.
7. Uncheck the "Use DDE" checkbox if it is checked.
8. Click "OK" and "OK" again.
9. Repeat steps 3-8 for the XLSX file type if you have Office 2007 installed.
10. Click "OK" to dismiss the Folder Options dialog.