A better way for handling this is the Avast BartCD, A livelinuxcd like
Knoppix or you have to build your own WindowsXP Bootcd by using Bart L.s
PE-Builder . But the Problem is that Avast will only work with the Avast Bart CD.
The Problem with a Bootdisk is, that is is almost a Dosdisk and so you are not able to access a NTFS Partition.