WIth Server 2008 R2, Plesk 9.5 has a bad habbit of not giving itself permissions to the C:\Windows\Assemblies folder. Due to this, ASP and ASP.NET applications will not function correctly. In order to fix this you must follow the steps below.

With Windows Explorer, browse to C:\Program Files (x86)\Parallels\Plesk\etc\DiskSecurity\DiskSecurity.xml
Right click on DiskSecurity.xml and edit the file with Notepad or another plain text editor.
Insert the following line at the bottom of the file, BEFORE </Entries>:

<
Entry AccounType="1" Account="Psacln" Path="C:\Windows\assembly" AceFlags="FilesOnly" AccessMask="Read" EntryFlags="0x0" />

Save and close the file.
Open a CMD Prompt and navigate to C:\Program Files (x86)\Parallels\Plesk\admin\bin
Once you are at the root of the foler type the following command:

ApplySecurity --apply

Execute the command, open Task Manager, look at the Processes tab, and find ApplySecurity.exe
Wait for the process to terminate, close task manager.
Stop Plesk, Reset IIS, Start Plesk

Verify that ASP is now working correctly by visiting an ASP Page or ASP based application hosted on the server.