This is an issue with your operating system, actually, not OOo. The operating system should associate a specific program with a specific file extension. You need to tell Windows XP that for files of type .doc and .xls it should fire up Writer or Calc as appropriate.
Running this utility should set the associations correctly: OOoAssociation Utility
If this has answered your question please go to your first post
and use the Edit button, and add [Solved] to the start of the title. You can use the green tick icon at the same time if you like.