Like Carl said, OO is free and works very similar to Excel. It is not 100% compatible with Excel, buy usually close enough.  The Works files ... I'm not sure.  When I bought my first XP machine I made it plain to the sales rep I had to be able to open and use old Works files.  "Sure, no problem, buy Excel"  I did.  Good program, would not open he Works files I had.  I eventually spent $50-$75, whatever it was to buy a genuine copy of Works.

I think ... that is the warning ... you can  use Works files with Excel or Open Office if they was saved with the right extension.  But if not, I'm not so sure.  I'd have to go play with a few to say.