If you are having trouble opening any Microsoft Office apps — Word, Excel, Powerpoint — on your new OS X Lion installation the problem is most likely that you have Office for Mac: 2004. One of the main shifts from Snow Leopard to Lion is that the latter does not support what is known as PowerPc apps, and significantly that includes the 2004 edition of MS Office.

If you want to stick with Lion you have three options:
 
1. Upgrade to Office 2008 or 2011
The simplest solution, but of course it adds to the cost of the Lion upgrade.
 
2. Download the free Office suite LibreOffice
LibreOffice is not only free, open source software, but it will both import and export to all Office formats. The interface may look a bit dated, but the functionality is just as sophisticated and arguably even more powerful in some respects than Office.
 
3. Buy Apple’s equivalent software from the App Store
Apple offers ‘Numbers’ (for spreadsheets), ‘Pages’ (for word processing) and ‘Keynote’ (for powerpoint) as their own alternatives to Microsoft’s signature software package. These will all import your old 2004 Office files and export to MS format. Currently these are retailing at $19.99 each in the App store, so if you only use one of the MS programs this could be significantly cheaper than upgrading to the latest version of Office. You’ll also get Autosave, Versions and full screen functionality built in.

If you are loathe to part with your Office for Mac: 2004 for some reason, then the fourth option is to revert to Snow Leopard. This should be possible so long as you didn’t buy a new Mac that came pre-installed with Lion.

Related articles:
running PowerPc apps on Lion
will my old apps work with Lion?
how to turn a Lion into a (Snow) Leopard

featured picture: Office for Mac alternative icons by deleket