I am attempting to install SharePoint Portal Server 2001 on a Windows 2000 Advanced Server (SP4). I have successfully installed it onto a similar server for testing purposes but in this case the installation fails with the following message in the errorlog:

SharePoint Portal Server: [2] CVAIComponent, Error 0x80070426: VAIFy failed
SharePoint Portal Server: [0] CVAIComponent::Install, Error setting up VAI.

Having scoured the Microsoft Knowledge Base, TechNet, MSDN and a number of SharePoint sites I have attempted the following:

- Listed all Microsoft hotfixes on the machine, none of which claim to conflict with SharePoint but have removed all of them and attempted the install again.
- Checked that no host header is specified in the default web site properties (set to All Unassigned and Port 80)
- Checked that localhost on port 80 is a valid way to access the default web site
- Verified that MDAC is installed correctly (version 2.7) using Microsoft's component checker
- Checked that at least one drive letter is free for installation (drive M is not currently used)
- Exchange Server was not previously installed on the machine
- The date on the computer matches the date on the domain controller
- "Cache ISAPI Applications" is enabled
- The World Wide Web Publishing service is enabled and running
- NT LM Security Support Provider is enabled and running
- Net Logon is enabled and running
- IIS Admin is enabled and running
- Event Log service is enabled and running
- Protected Storage service is enabled and running
- HTTP Keep-Alives are enabled on the default website
- Distributed Transaction Coordinator is enabled and running
- RPC is enabled and running
- I have disabled Virus scanner during installation
- The server is not logged on using cached credentials
- There does not appear to be any COM problems on the machine
- The event log does not mention any problems with duplicate SIDs
- No Office Server Extensions are installed (FrontPage Server Extensions are installed but they are also installed on the test server and this caused no problems there).
- The server is not in a clustered environment
- The server was not upgraded from Windows NT
- SMTP service is installed and running

The following programs are installed on the server
- Backup Exec Open File Option
- Dell OpenManage Array Manager
- Deskartes Domain Knowledge v3
- EditPlus 2
- Intel (R) Pro Ethernet Adapter and Software
- Intel (R) ProSet II
- InternetDesk
- Java 2 Runtime Environment Standard Edition v1.3.1
- JRun 3.1
- LiveReg (Symantec Corporation)
- LiveUpdate 1.6 (Symantec Corporation)
- McAfee NetShield 4.5.0
- Microsoft .NET Framework 1.1
- Microsoft .NET Framework SDK (English)
- Microsoft Mobile Internet Toolkit 1.0
- Microsoft SQL Server 7.0
- Microsoft Visual Studio 6.0 Enterprise Edition
- Microsoft Web Publishing Wizard 1.53
- MSN Messenger 5.0
- NETG Software
- ON Command Remote Host
- SharePoint Portal Server (obviously not working!!!)
- Skill Builder DX
- Skill Builder DX Administration
- Sunrise Enterprise
- Sunrise Enterprise Database
- Symantec PCAnywhere
- Terminal Services Client
- Timecard
- Veritas Backup Exec
- Web Client
- Web SkillVantage Manager
- Web SkillVantage Reports
- Winzip

I have inherited this server and it already has a number of applications running on it (mainly web based).

Having attempted to repair, uninstall and reinstall numerous times I seem to have come to a dead end and am not sure what to try next - any suggestions??