I recently had an issue with a thrid party vendor where they asked me to uninstall Adobe Acrobat, then reinstall it again from their CD. The installation kept failing. When we unpacked the install files and ran the program manually, we got an Invalid M: Drive, which was a mapped network drive. With SBS, folder redirection was enforced, so the My Documents folder had been redirected to the M: drive. Some applications do not like this and will fail with an error. Often, this is caused by an incorrect setting in the User Shell Folders registry key.
Fortunately, there is a Fit It solution for this. Microsoft article KB886549 provides a Fit It resolution that will restore the User Shell Folders back to default settings. This article is found here – http://support.microsoft.com/kb/886549
Note: You will need to reapply the folder redirections again after this Fix It is run.