Thanks to everyone for their help. I have had some success in getting things back and working. I found these on the web which were useful:

http://support.microsoft.com/kb/224071

http://support.microsoft.com/kb/822852

These are the steps I took to get my DTS jobs back:

- Copied the existing SQL Server 2000/2005 Data directories to a backup location

- Uninstalled SQL Server 2005 (all components)

- Uninstalled SQL Server 2000 (all components)

- Restarted

- Reinstalled SQL Server 2000 (everything running under Local System account - mixed mode)

- Stopped SQL Server Service and SQL Agent Service

- Started SQL Server with the following via CMD window:

C:\Program Files\Microsoft SQL Server\MSSQL\Binn>sqlservr -c -m -T3608

- Dettached the MSDB database using the following:

use master

go

sp_detach_db 'msdb'

go

- Copied in my old MSDBDATA.MDF and MSDBLOG.LDF files into the following directory (renamed the newly installed versions as backup):

C:\Program Files\Microsoft SQL Server\MSSQL\Data\

- Stopped SQL Server single user mode (CTRL-C in CMD window)

- Restarted SQL Server normally

- Ran the following attach script:

use master

go

sp_attach_db 'msdb','C:\Program Files\Microsoft SQL Server\MSSQL\Data\msdbdata.mdf','C:\Program Files\Microsoft SQL Server\MSSQL\Data\msdblog.ldf'

go

Hey presto, my DTS jobs have reappeared!

Thanks again.

Now to see if I can attach the other user databases...