I was having major speed issues with Excel 2007 on my PC (2.0Ghz Core 2 Duo,
2G RAM, XP Pro). Whenever I would save a .xlsx file down to an .xls file, or
try to work within some .xls files, EVERYTHING took a long time - cut/paste,
formatting, undo... it was ridiculous (and still is!). So, the secret? Open
up a brand new .xlsx file, copy the data from the .xls file (or slow .xlsx
file), and paste it in the new file. Presto! Things are much faster. Even
when I tried to save an .xls file as a .xlsx file, it was still slow, but
this fixed it.

Beware that if you try to save this as a .xls and work in that, it may slow
things down again.

ARE YOU LISTENING, MICROSOFT??