As we installed it so early, we left Office 2003 installed as well. We've run into some interesting problems that I believe are as a result of that.
The most irritating example is that when you open an Excel file attachment from within Lotus Notes, Notes hangs, Excel opens but without a file. Then after what seems an eternity (really less than a minute), it breaks loose and all is fine. My friend even discovered that if he clicked on the Open icon in Excel and then canceled the dialog box that Excel would take off. The same thing happens when you double click on an Excel file in Windows Explorer. I haven't noticed it for Word files.
I Googled this and found this blog entry.
That fixed it. Interestingly, we both noticed that the text in our entry read
"C:\Program Files\Microsoft Office\Office11\EXCEL.EXE" /eYou can just click on Browse in that dialog box and navigate to the Office12 folder.