Computer keeps making me reauthorize

[prebys]

I am syncing an iPhone 3G to a Windows XP machine. My kids both sync iTouches to the same machine, under different accounts. We synced older iPods before these. Everything has worked fine for several years. Now, however, it tells us to authorize songs and apps every time we sync. When I enter the information it tells me (duh) that the computer is already authorized and proceeds.

Any advice would be greatly appreciated.

-Eric