Well, the "Add or Remove Programs" dialog in XP is supposed to show how much each program is used in the last 30 days as "Rarely", "Occasionally" or "Frequently", unfortunately I have several lines that show up as "Occasionally" With a last used date of 1/9/2008 (nearly two years ago), so I wouldn't even put much trust in that data even if I could find a way to locate and extract it.
As far as an actual count I doubt it is even there, much less readily available