I'm sure anyone using Excel 2007 has run into this show stopping problem. You have a huge dataset, say something like 10k results or more (benchmarking data in my case). You format the data, and then put it into a graph. At this point Excel 2007 becomes slow. Sometimes it'll just crash. Either way it becomes unworkable with large data sets that posed no problem for Excel 2003. In an effort to solve the problem, I ended up having a hackish fix. I'd just do one operation on the graph, save the results, close, open the workbook up and repeat. Obviously this took time.
I was pointed to a fix by kellanved. Apparently Microsoft are aware of this issue and have made a hotfix for it. Since the hotfix hasn't been through the vigorous testing that they do *cough**cough*, you have to request the hotfix from MS.
Excel 2007 hotfix package: June 13, 2007 details the hotfix and when to request for it.
After installing the hotfix (which took MS a nice quick hour and a half to return my request), Excel flies. The large data sets have become manageable, no more hackish fixing to the problem.
So if you've got the same issue, give the hotfix a try.