The Windows Installer team has seen it in their heart of hearts to provide some information on where to receive professional support for Windows Installer.
Yes, I am being a little sarcastic. If I don't maintain a support contract with Microsoft, then I am looking at spending anywhere between $99 and $515 for professional support. I wish I had thought to charge $99 every time someone emailed me a question about Windows Installer (Note to self...). :-)
In lieu of spending your hard-earned money on professional support, I thought I would direct you to what can often be more valuable, community support. There are many great resources in the nebulous cloud of the internet where you can find the information that you need concerning Windows Installer. The advantage of paying for it I guess would be the instant gratification of having an answer right away. What is not guaranteed is how long you will be on hold waiting for that answer.
The following is by no means a comprehensive list, but I believe this list should be in everyone's link box:
Windows Installer SDK - SDKs are not always the easiest reading material, but this is by far the most comprehensive reference concerning Windows Installer.
microsoft.public.platformsdk.msi - The most active newsgroup discussing Windows Installer
microsoft.public.windows.msi - Another newsgroup resource, but not quite as active as the platformsdk group.
microsoft.public.win2000.msi - The least active Windows Installer newsgroup.
InstallSite.org - Excellent site run by Stefan Krueger that contains, tutorials, tools, downloads, forums, and more. The forums are an excellent place to ask questions and get help. InstallSite is a very active community. It is mostly InstallShield-centric, but Windows Installer is Windows Installer no matter what tool you are using.
InstallShield Forums - These are the official forums run by Macrovision, and this is also a very active community.
Wise Product Forums - These are the official Altiris (now Symantec) forums with yet another very active community.
wix-users Email Group on Nabble - Whether you are a user of WiX or not, this searchable archive of emails about WiX and Windows Installer contains an incredible wealth of information. If you would like to subscribe to the email list, you can do so here.
I am in no way putting down the Microsoft employees or their capabilities. I am sure they could provide you with very sound advice and help. However, if you don't need your answer yesterday, 99 times out of 100 the community will come through for you.
I tried not to duplicate Chris's earlier post too much. I wanted to focus more on where you can ask questions and receive answers. The community will appreciate it very much if you were to search for your question first to see if it has already been answered.