Nick,
Don't forget that the XP mode is just that - XP, it has all the security issue potential that the full version has ...
Run all the anti-malware that you would in a real XP in the Virtual one - unless of course you are using the Virtual XP as a test environment
Saying that Backups that use compression can play all sorts of tricks on Heuristic Scanners - better to be safe than sorry though - boot up your live XP mode do a full deep scan from inside the virtual machine and when your sure that it is clean replace the backup