Yes- this is a real bummer and has caught out many others. although the Apple fanboys will tell you (unhelpfully) that you "should have known".
At this point your options would be
1. Assuming you have a clone of your old system, go back to Snow Leopard
2. Buy MSOffice 2011 for Mac
3. Buy iWork, which should be able to open your Excel and PPT files although this may be a bit dicey
4. Download LIbreOffice - which is free and is quite happy opening MS Office files including Excel and PowerPoint. This is probably your best option.
I hope this helps.
I felt your pain when I purchased the mid-2011 Mac Mini to replace a dead iMac G5 last year, without doing my normal due diligence on Lion.
Like others I had the need to continue to access my finances in Quicken Deluxe 2002 (since rendered moot by Intuit's decision to release Quicken 2007 for Lion) as well as certain Excel 2004 charts that get trashed when opened in Excel 2011.
Since necessity is indeed the mother of invention, I worked out this solution: I installed Snow Leopard (with Rosetta) into Parallels 7 in Lion:
[click on image to enlarge]
Full Snow Leopard installation instructions are available here: