Hello Jing,
I do not see the snapshot but if the VM hard disk is present then the problem might be with the guest OS.
Try the following steps:
1. First of all, try accessing the hard disk via Parallels Explorer. If you would be able to see the content of the disk then it is not corrupted.
In any case, back up the hard disk, just copy windowsxp.hdd to some other place.
2. If there are any snapshots in the hard disk (if you right-click on windowsxp.hdd and see several .hds files inside), try merging them using Parallels Image Tool.
3. After that, I would suggest to try Windows repair installation as per article
http://kb.parallels.com/en/5138
That way we would also be able to check whether Windows detects any OS inside or not.