Can I change the PATH environmental variable in Windows 7 without restarting?

The requirement really isn't "must restart", it's "must log out/log back in" -- mostly for your current user processes to pick up the change.

If all you're interested in is for the variable changes to take effect in a command window or power shell, starting a new shell after making said changes should be all that's needed.

In my Win7 (64-bit) the console command

path=drive:\dir;%path%

adds drive:\dir to the beginning of the current console's path without starting a new console. No idea why your Win7 should behave differently. I'd like to know how some programs add themselves to the %path% variable for every instance of the console, though.

To change the Windows 7 path manually, check: http://geekswithblogs.net/renso/archive/2009/10/21/how-to-set-the-windows-path-in-windows-7.aspx