Hello Martin,
distributed transaction is managed through the Microsoft Distributed Transaction Coordinator (MSDTC)
You can start by reading on this subject at :
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/architec/8_ar_sa2_0291.aspI've done this a few years ago in VB6... This is fairly straightforward. What you have to do is start MSDTC (which might be stopped by default) and activate "Transaction Support" on your COM+ component... MSDTC will automatically take care of the transaction, whether it's on the same server or on different servers.
Hope that helps,
Alex