If you are installing a program and receiving a pop-up message during installation of valid short file name this applies to you. I removed a drive from my computer and had this happen to me.

Steps to Fix “not a valid short file name”

1. Click Start, and then click Run.

2. In the Open box, type regedit, and then click OK.

3. Under Registry Editor, locate the following registry key: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders

4. On the right pane, note the values in the Data field of each entry. If any value contains a drive that is not correct for your computer, right-click the entry, type c:\my documents in the Value data box, and then click OK.

5. Repeat step 4 for each entry whose Data value contains an incorrect drive.

6. Repeat steps 3 through 5 for each of the following registry keys:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders

7.Close Registry Editor.

Re-run your setup program and you shouldn’t see “not a valid short file name” error. Also, I’d recommend running the registry cleaning portion of CCleaner and checking your Environmental Variables (Under System Properties, Advanced, then Environmental Variables.)