Reducing virtual disk size

I am using Parallels desktop 4.0 for MAC (OS X) (Intel) with Windows XP installed as a guest OS (no other OS installed). When I first started a couple of months ago I needed more space available for Windows so I expanded the virtual disk capacity to 100 GB. No difficulty with that. Now, however, I want to go the other way and reduce the capacity back down to 50 GB. I have been just going around in circles. The instructions say to use Parallels Image Tool to compress the virtual hard drive. I did that twice but when I check the .pvm file with finder it tells me it's still 99.71 GB, no change from what it started out as. When I'm in Windows the C drive shows about 15 GB used with a capacity of 100 GB. The virtual drive shows as an expanding type but yet appears to stay fixed in size. Is the finder info lying to me, does it always show the max capacity rather than what is actually being used??? Is there some way to reduce the capacity of the drive? I saw somewhere that there is software available to change the partition of a drive but I did not think that was what I needed. I know very little about partitions but it looks to me like it is set up as a .pvm file not a partition. There was another post asking pretty much the same question but the Parallels Team response was to give a link explaining how to compress the drive which they already said they had done and so have I. Any suggestions other than compress the drive?