I have had similar problems with paths that contain spaces, and i usually end up using the DOS names for the directory, just to be sure, maybe :

$VSS = 'D:\progra~1\micros~3\common\VSS\win32\ss.exe';
#...
system($VSS,'ss','checkout -I-');
[download]

I also moved the parens on system, because i thought it made cleared the precedence you wanted, but i don't suspect that is a cause for error. If you still suspect it, you could use fork() and exec() yourself.

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i had a memory leak once, and it ruined my favorite shirt.