Citrix has released a new hotfix rollup pack for XenApp 5/Presentation Server 4.5 for Windows 2003.
Before you install this latest rollup pack make sure you read through this article http://support.citrix.com/article/CTX119069
One of the most important prerequisites is to update your Citrix Licensing server to 11.6 https://www.citrix.com/English/ss/downloads/details.asp?downloadId=1688507&productId=186%20target=
If you have not yet installed hotfix rollup pack 3 then you first need to install the Microsoft Visual C++ 2005 SP1 Redistributable Package from here http://www.microsoft.com/downloads/details.aspx?familyid=200B2FD9-AE1A-4A14-984D-389C36F85647