Well, reading all the log I tried to understand what was going on. It happens that on disk4s1 there is no Windows installed. So I think that the creation of the VM aborts.
I tried to arrange yet another USB disk with a Windows XP already installed, which I managed to do. I plugged it in and started Parallels Desktop. I was then able to creat a boot camp virtual machine (...). So I tried to follow the directions on your first post.
Unfortunately, I got stuck right on the 3rd <Storage> section, that doesn't exist on the the DiskDescriptor.xml file.
This is my DiskDescriptor.xml file:
	Code:
	<Parallels_disk_image Version="1.0" >
    <Disk_Parameters>
        <Disk_size>156301488</Disk_size>
        <Cylinders>155061</Cylinders>
        <Heads>16</Heads>
        <Sectors>63</Sectors>
        <Padding>0</Padding>
        <Miscellaneous>
            <CompatLevel>level2:p</CompatLevel>
        </Miscellaneous>
    </Disk_Parameters>
    <StorageData>
        <Storage>
            <Start>0</Start>
            <End>1</End>
            <Blocksize>1008</Blocksize>
            <Image>
                <GUID>{5fbaabe3-6958-40ff-92a7-860e329aab41}</GUID>
                <Type>Plain</Type>
                <Flags/>
                <File>PhysicalMbr.hds</File>
            </Image>
        </Storage>
        <Storage>
            <Start>63</Start>
            <End>78156224</End>
            <Blocksize>1008</Blocksize>
            <Image>
                <GUID>{5fbaabe3-6958-40ff-92a7-860e329aab41}</GUID>
                <Type>Bootcamp</Type>
                <Flags/>
                <File>/dev/disk5s1</File>
            </Image>
        </Storage>
    </StorageData>
    <Snapshots>
        <Shot>
            <GUID>{5fbaabe3-6958-40ff-92a7-860e329aab41}</GUID>
            <ParentGUID>{00000000-0000-0000-0000-000000000000}</ParentGUID>
        </Shot>
    </Snapshots>
</Parallels_disk_image>
 And this is my Partition Inspector data:
	Code:
	*** Report for internal hard disk ***
Current GPT partition table:
 #      Start LBA      End LBA  Type
 1             40       409639  EFI System (FAT)
 2         409640    625140295  Basic Data
Current MBR partition table:
 # A    Start LBA      End LBA  Type
 1              1       409639  ee  EFI Protective
 2 *       409640    625140295  07  NTFS/HPFS
MBR contents:
 Boot Code: Unknown, but bootable
Partition at LBA 40:
 Boot Code: None (Non-system disk message)
 File System: FAT32
 Listed in GPT as partition 1, type EFI System (FAT)
Partition at LBA 409640:
 Boot Code: Windows NTLDR
 File System: NTFS
 Listed in GPT as partition 2, type Basic Data
 Listed in MBR as partition 2, type 07  NTFS/HPFS, active
 What changes should I make to diskdescriptor.xml?
Update
I was able to update both files with some data calculated with your instructions. I updated the 2nd Storage section in DiskDescriptor.xml with that data, even though it points to a PhysicalMbr.hds file and not to a PhysicalGptCopy.hds.
This way I was able to start the Virtual Machine, which triggers a configuration step "Please wait while your virtual machine is being configured... [..] (Step 1 of 4)". In the middle of Step 1 it says that I have to manually do something on the virtual machine (You need to manually continue the configuration process.). 
It then tries to boot the VM but it stucks with the following information:
	Code:
	Parallels(R) VGA-Compatible BIOS Version 3.0.2111.89721
4.0.3810.351321 Fri, 23 Jan 2009 16:33:16
2048 MB Physical memory installed
SCSI controller is not installed.
Network bootrom is installed.
Trying to boot from Primary Master IDE drive ...
boot0: MBR
boot0: MBR
boot0: error
 And I am stuck again. I can imagine this is because my USB disk was MBR partitioned and disk0 is GUID partitioned...
How to proceed?
Thanks in advance
RobS