Tell us a little bit more about what version of Windows & Outlook you're using. And do you have more than one version of Microsoft Office installed (2003 and 2007, for example)?
I've still got a few Apple support articles you'll want to look over to see if they help.
If you're trying to sync with an Exchange server look here:
Otherwise here is basic troubleshooting:
And this is advanced troubleshooting:
Let us know if this helps out at all.