Because of differences in functionality, I use both VPC and Virtual server for most of the same virtual machines. Today on my vista I ran into something I could have expected, but for some reason refused to think about until present: permissions on the folder where your VPC's are located.
Because of the UAC functionality (I decided to try this out for a while) I am logged on using normal credentials. in Virtual server I am Just a user, no more. For that I use the "run as administrator" feature when starting the admin website. No problem I thought. BUT...
As I now realize, it is not only for the service I am a normal user, but these permissions are also set on the folder you enter in the virtual server, server paths setting. It had my account added as just for read, no write. so far so good, because the administrators have full control on the folder. BUT now I tried to start a virtual machine in VPC2007 by just double clicking the vmc file. and guess what: it told me, the vpc could not be started because the vhd file was read only or in use by another process. So I checked the attributes of the file: no Read Only attribute set. I opened up OpenFileView (a tool that checks which process has a lock on which file) and low and behold, no file handles open to the vhd. It took me some time to realize the reason for this "read only"... I edited the settings of the folder (through the virtual server webinterface) and everything is up and running again. MAN did I feel stupid. It took me about half an hour to realize what was wrong... Let's blame it on Friday...
Addition: a colleague of mine (Bart) alerted me that I did not post the real solution. I did post it between the lines, the solution is to give your own account at least modify permissions through the Virtual Server website.