Basically, I followed dmanmn's formula. I'm running an iPhone (using 2.0 but not 3G -- mine's still on order), MacBook Pro (Leopard), Parallels, Windows XP (sp3) and Outlook 2007. I've spent hours with Apple Tech Support (useless), uninstalled & reinstalled, (also useless). But dmanmn's recipe worked for me. I think the order of the steps is what's critical:
1) Backup Outlook (who can afford to loose 1400 contacts?)
2) Uninstall iTunes
3) Uninstall Apple Update
4) Uninstall Apple Mobile Support
5) Delete C:\Documents and Settings\Administrator\Application Data\Apple Computer\SyncServices (and all files in that directory -- yours may not be in the \Administrator subdirectory)
6) Reboot
7) Reinstall iTunes
8) Start iTunes and plug in your iPhone
9) Here's the key -- I got the dreaded "Unable to load provider data..." so I quit iTunes and was about to try the whole freakin process again, but remembered that I should probably reset the sync history (Edit > Preferences > Syncing Reset Sync History), so I restarted iTunes intending to do that and...
THERE WAS NO "Unable to load provider data..." message on the second start of iTunes, and...
ON THE INFO TAB, Outlook showed up in the drop down boxes and all my email accounts showed up as well.
10) I always set iTunes for manual sync in case I want to change a setting, when I selected SYNC, it told me that my iPhone was set to sync with another profile and asked if I wanted to reset it to sync with the current profile. I selected YES
11) iTunes then gave me the choice of merging data or overwriting the iPhone with the data on my computer... I chose the overwrite.
12) The sync started and ran all the way through successfully (Outlook contacts, calendar, 4 mail accounts as well as music, photos).
13) Just to make sure I wasn't a "one sync wonder", I disconnected, shut down iTunes, rebooted Windows, restarted iTunes and plugged in my iPhone and...
IT WORKED AGAIN (another big Woo Hoo).
This time it told me there were purchased files on the iPhone that were not in my iTunes library and asked if I wanted to sync those. With great fear I selected YES and it then sync'd the Apps I've bought since the App Store opened.