Proposed Method  
1.Backup

1.Back up data on external hdd.

2.Back up Firefox extensions with FEBE extension onto ext HDD.

3.Backup Firefox Bookmarks with Foxmarks extension onto Foxmark's online storage.

4.Backup Firefox profile with Mozbackup onto Ext HDD ~ http://mozbackup.jasnapaka.com/download.php

1.Reinstall

1.Boot from Partition Boot CD (Eg Ultimate Boot CD - with Cute Partition Manager)

2.Partition HDD (Eg using Cute Partition Manager: 120 GB HDD - 2 partitions first 30gb, second 90gb)

3.Restart PC booting from XP install disk.
4.Load XP onto 30gb partition.

1.Loading Security
1.Install Returnil, AVG, Online Armor, Sandboxie, Firefox.

2.Move "My Documents" to the Data Partition. ~ http://www.mydigitallife.info/2007/1...-xp-and-vista/

3.Before running Firefox install and run Mozbackup to move profile to Data partition - http://lifehacker.com/software/ask-l...sta-257297.php

4.Put a short cut to sandboxed Firefox ~ http://www.sandboxie.com/index.php?ConfigureMenu#shell

5.Edit the Sandboxie Configuration to allow changes to Firefox (my own personal preference and not very security-minded but it's one place where I might have to compromise)

If anyone's got any tips, changes or recommendations I should add, please let me know. I'll try and update the OP so that it's as close to ideal as possible.