You can freeze windows by selecting a row, column, or cell. If you select a row or column, it will freeze everything to above or to the left (respectively) of where you selected. If you pick a cell, it will do both.
You can also split the page, if you like. It basically gives you two spreadsheets over each other. This is generally useful for comparing two distant parts of a spreadsheet, so it might work for what you want, too.
Both commands are located under the Window menu.