If cancelling the send and receive and then deleting from outbox doesn't work it must be because your cancel instruction isn't working. it will not let you delete an email that has started to send. You will need to find another method for cancelling the "send and receive"
As rojash said more or less but in Outlook, Tools, options, mail setup, and uncheck the box for "send and receive immediately when connected"
It's also possible that it won't send if you created the email in Outlook with one account but are trying to send it with another. On your Send and Receive button at the top, click the down arrow next to it, send and receive settings, and disable temporarily the scheduled Send/Receive.
Don't forget to undo all this after you've managed to delete the email from the outbox.
For Chris' info, on my Vista Home Premium, if you search for .pst files with hidden folders showing you don't get the results you would expect. Ythe only one it finds on my syste is the Outlook.pst which you can only open with outlook. You cannot seperately view the pst files with Outlook closed. Least ways, i haven't found a way to do it.
Good luck Cheeky, you'll get there in the end!