I have Windows 7, Office 2010 suite on a new laptop, and same problem on a desktop with Office 10 running on Vista.
On the Windows 7 laptop, I copied in my .pst file from my old computer, which was running Vista and Office 2007. Once I did that I wanted to set my Calender reminder default setting from 15 minutes to 1 day. I found where to make the change (File-Options-Calendar Options), but the change doesn't save. I click to change the default reminder to anything other than off or 15 minutes and it goes right back to 15 minutes as soon as I close the options window.
The other computer was a straight upgrade to Office 10 from Office 07 and it has the same problem.