If you want a space as thousands separator in all Windows applications:
- Click Start > Control Panel.
- Click Clock, Language and Region.
- Under Region and Language, click Change the date, time or number format.
- Click Additional Settings...
- Click in the Digit grouping symbol box.
- Type a space (you can't select it from the dropdown, but you can type it in yourself).
- OK your way out.
If you want to use space only in Excel, while keeping the standard thousands separator in other applications:
- In Excel, click File > Options.
- Click Advanced.
- Under Editing Options, clear the check box Use system separators.
- Click in the Thousands separator box.
- Enter a space.
- Click OK.