iphone syncing

You can use the same computer but you need to use different login accounts. That will keep your address book's separate.

One thing that might help.

Another login as suggested by Jeff, might be the way to go.
You can use another computer to if you want.

If you do use another computer or different login, make sure to open itunes and the contact manager first.
Then enter at least one contact into your contact manager.
Then you can connect the iphone to sync your contacts.
Also, if you sync calendars, make sure you do the same by entering at least one calendar event first before syncing your iphone.

Hope this helps.

I could be wrong but I think this can only happen if the two of you are also sharing a MobileMe account and are using it to sync contacts. If so, you can upgrade MM to a family pack which will give each member of your family separate storage. I had to do this recently so that my wife and I could both sync contacts without interfering with one another.

Actually I may have misread your answer. You said that you both have separate iTunes accounts. That's not what I meant. You need to have separate XP accounts and log into your computer separately. Are you doing that?