Can somebody please help???

I have a Windows XP machine, on which I am running Virtual PC 6. For my various labs I have created multiple virtual machines, with some multiple disks. The main problem is that when I run the VhdResizer > Open the *.vhd file, it gives me the following error

"Invalid Vhd file, please select another".

The main problem is that for some VHD files, its working and for some it is not! Can somebody tell me some trouble shooting steps? I have scanned the whole forum and also googled it a lot, but was not able to come up with any kind of solution with Windows XP (for Vista there seems to be a lot)

Thanks.