When you build or rebuild the solution, there is a log of the build output, including the post build events. If you look at the output from the xcopy commands, it may help you troubleshoot your commands.
It's been quite a while since I looked at the Guestbook demo setup, but I think the copy commands should look something like this:
xcopy /s /y "$(ProjectDir)bin\Guestbook.UI.dll" "$(SolutionDir)Web\bin\"
xcopy /s /y "$(ProjectDir)bin\Guestbook.Business.dll" "$(SolutionDir)Web\bin\"
xcopy /s /y "$(ProjectDir)bin\Guestbook.Data.dll" "$(SolutionDir)Web\bin\"
xcopy /s /y "$(ProjectDir)Guestbook\*.as?x" "$(SolutionDir)Web\Guestbook\"
I hope that helps,
Jamie