That version of classic isn't supported with 2008 and I don't think runs with IIS7 either. Could you virtualise an older server or migrate to a more modern version of the product?
Our documentation suggests otherwise, stating that Windows 2008 Server is supported.
We installed it on a new Windows Server 2008 (32bit, non R2) instance. Apart from the Internet Edition not creating correct timestamps when a new incident is entered, it appears to be working fine.
Let's compare notes then. For HD Classic version 7.2.6 the HelpDesk Supported Platforms guide lists the OS platforms for the server component on page 7. That doesn't list 2008. The highest it goes up to is 2003 R2 64 bit edition. If you mean 7.2.6 of ServiceDesk, that is a different product and I'd ask your question in that forum.
FYI - we managed to complete the install and now have a working system (apart from Change Manager).
A great deal of attention needs to be given to give permissions and IIS7 but it is possible.
Do you have any details of the exact permissions that you needed to set in IIS?