Type the first column of numbers into Excel, column A. Then copy them into column B.
Highlight column B.
From the menu, choose Format, Cells, Custom (at the bottom). If you are using Excel 2007, right-click the selected column, choose Format Cells, and then choose Custom at the bottom of the list.
In the Type box, delete the current entry (the default entry is General).
In the now-empty Type box, enter this sequence:
The first model above, before the semi-colon, refers to the presentation of positive numbers; the second (red) model refers to the presentation of negative numbers. You can remove the [Red] if you don't want red in your worksheet.
Make sure you include the zeros, decimal places, and don't forget the commas at the end of both the positive and negative sequences.
Press OK. The numbers are now nicely rounded to the nearest thousand.