If you installed Microsoft Word it should of set itself as the default for .doc files. Right click on the document you are trying to open. In the menu there should be a "Open with" selection. Click it and then click "choose default program." Go to C: -> program files -> Microsoft Office -> Office 12 -> Word (word.exe).

If that doesn't help try running the Office Diagnostic tool. Goto Start -> All Programs -> Microsoft Office -> Office Tools -> Microsoft Office Diagnostics