On Windows 2003 Server, you may encounter the error 26005: Could not start IMA Service in CTX_MF_IMA_StartIMAService State = 1 during Citrix Presentation Server 4.5 installation.
http://support.citrix.com/article/entry.jspa?entryID=11800 addresses the problem, but does not seem to resolve.
After testing, it seems that IMA will not start under the Network Service account but will successfully start with the Local System account.
To prevent the install from failing click Cancel when the 4.5 installer displays Registering New Services. Do NOT click yes when it prompts "Are You Sure".
Open Services and change the logon account of the Citrix IMA service to "Local System" from "Network Service".
Then click cancel on the "Are You Sure" prompt to resume the installation.
The installation will then complete successfully.