TRYDL, thanks for the reply.
I found the answer on Altiris Knowledgebase in aritcle id: 1062
https://kb.altiris.com/display/1/kb/...aid=1062&n=1&s=
Which outlines my problem specifcally with the inventory pages and states the cause as being:
The virtual link used by Software Inventory Solution in IIS has had the "Anonymous Access" box enabled either directly, or unintentionally by way of filtered-down changes made to a parent higher up. The only authorized access method is through Windows Authentication, so having the anonymous access box enabled causes the error to be generated.
And resolution being to uncheck anonymous if it's checked.
I did that and all is working fine again.
Mike