When I try to start Slicer on my Windows system, I get an error similar to one of the following:

• Error: couldn't execute "C:\Documents": invalid argument
• Error: couldn't execute "C:\Programs": invalid argument

This is a problem with some older versions of Slicer. There are no provisions in place to deal with spaces in the directory path leading up to the slicer2 directory.
More recent versions have fixed this problem for Windows by using the canonical short name of the directories (namely, C:\PROGRA~1) to avoid the Tcl parsing problems that occur with unexpected spaces in the directory path strings.
There is no current fix for UNIX pathnames that include spaces, as this is something that generally causes problems, and should be avoided.

To solve this problem, either move the slicer2 directory to a new location, such as c:\slicer2 (with no spaces in the directory names leading up to it), or obtain a newer version.