From open office help:
Changing the Association of Microsoft Office Document Types
To change the association of Microsoft Office file name extensions to open the files either in OpenOffice.org or in Microsoft Office, using Microsoft Windows:
1.Exit OpenOffice.org and the OpenOffice.org Quickstarter.
2.Run the OpenOffice.org Setup program:
Click the Windows Start button, choose Control Panel, click Add or Remove Programs, select the OpenOffice.org 2.3 entry, click Change.
3.In the OpenOffice.org Setup, check the Modify option and click Next until you see the file types page.
4.Select or deselect the file types. Click OK and complete the OpenOffice.org Setup program.