A hotfix is a special build for specific customers to fix a specific problem. Microsoft recommends that most hotfixes be applied only to systems that are experiencing the problem. Hotfixes are available free of charge for those fixes that solve your issue. You won't have to wait on hold with everyone else waiting for assistance to get it.
NOTE: If you are in need of virus or security patch-related support, call (866) PC SAFETY for free support in the United States and Canada. For other locations, contact Microsoft offices worldwide.
By far, the best and most efficient way to get a hotfix is to find the Knowledge Base (KB) article that refers to Microsoft Product Support Services (PSS) for more information or for the supported hotfix. An example of this can be found in this KB article: 833745 - Exchange Server 2003 Always-up-to-date Notifications do not work. If you were experiencing the problems in the article, you would get to the resolution section and it would tell you that Microsoft has corrected this issue in a supported hotfix and that you should call Microsoft Product Support Services (PSS).
When you click on the link in the KB article that tells you how to contact Microsoft Product Support Services, you’ll see a link that says “Phone Numbers, Support Options and Pricing”. This link will then take you to a product selection page, where you’ll select your product and be brought to the appropriate products support options page. In this case, here’s a sample of the Exchange 2003 Standard Edition Support options site. On this site you’ll find the proper support phone number for the type of customer you are.
At that point, you’d simply call the Toll Free number listed under the "Phone Support" section. Within the telephone menu options, you’ll be asked if you are calling for a hotfix. Simply press the appropriate menu item number. You will be connected to a person who will be able to retrieve the hotfix that you’ve requested and send you a link to download it from the Microsoft site. It’s just that easy and it’s free of charge.
All customers are eligible to receive a hotfix until the hotfix reaches the end of its availability date or until the parent product reaches the end of its lifecycle. Certain hotfixes are available after the standard hotfix availability date if a customer has signed up for extended hotfix support.
If you find that the hotfix you used does not solve your issue and you want to continue working with Microsoft to resolve the issue, you will be required to open a support incident and be subject to any charges related to that incident.
Also see: 328839 HOW TO: Apply Exchange Services Packs and hotfixes