Another possible issue is the USB transfer and the potential data loss (depending on the camera type) due to extra software layers (linux -> virtualbox -> winXP).
The cameras without RAM tend to just dump data without synchronisation and you may need to slow down the transfer rate. I have tried similar setup with Virtualbox and Win7 and had lines in the images or even completely corrupt images. Try taking some images during daytime, if you get data loss it will be more easily identifiable in the images.
By the way, see
here for how to get ASCOM running on XP.