Ok, I will try to explain what I am trying to do.
I coded a software in C# which uses the COM (Yahoo! Messenger TypeLib) type library of messenger to get the address book contacts for yahoo messenger. It looks something like this:
Messenger2Class messengerClass=new Messenger2Class();
messengerClass.AddressBook(ref addressBook);
IAddressBook book = (IAddressBook)addressBook;
entries = (IAddressBookEntries)book.Entries;
if (entries == null)
throw new Exception("Unable to load entries!");
foreach (IAddressBookEntry entry in entries)
{ //do something here; }
Now, this works perfectly when the address book is accessible. But if I uninstall Yahoo Messenger and install a new version (even the same one) the addressBook object simply returns null. And this happens with any version of messenger (above 8). Also on certain machines I have the same problem. On some of them works just fine on other doesn't at all. I tried to add a ABook key to the Yahoo registry but same result. If I use system restore and restore my machine to an earlier time when it worked, everything works fine again. So, if anyone experimented with this or have any ideas please help. Thanks in advance. Oh, and if you want to check it out, or check out the source code
http://ybuddyspace.blogspot.com 
