Simulating the Windows key on an old keyboard

Remap your capslock key with a quick registry hack? The Caps lock key is worthless.

Paste this into a file called something like remap_caps_to_windows.reg and double click it. You must restart for the changes to take affect

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout]
"Scancode Map"=hex:00,00,00,00,00,00,00,00,02,00,00,00,5c,e0,3a,00,00,00,00,00

CTRL-ESC brings up the Start Menu.

Unfortunately, it does not allow the other windows key shortcut functions. For instance CTRL-ESC-Pause/Break will not bring up system properties. And CTRL-ESC-R will only bring up Run if you release CTRL-ESC before striking R, and then it'll only work if there aren't any entries in your Start Menu beginning with "R".

In Windows 10 the keymap suggested in the accepted answer maps Z and M to B and maps B to Z.

It was funny until I realized my username has an "M" in it. Thankfully Win10 has the on screen keyboard option in the accessibility options during login, but this wouldn't show up for the "Run as administrator" dialog.

Anyway, using SharpKeys I found the correct map for CAPS LOCK to Win is this:

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout]
"Scancode Map"=hex:00,00,00,00,00,00,00,00,02,00,00,00,5b,e0,3a,00,00,00,00,00

You could create a custom keyboard layout with the key remapped.

Microsoft Keyboard Layout Creator may be able to do the job.

You can remap any key to be a Windows key using KeyTweak.

It's just a GUI interface to the registry, so does the same thing as what Zoredache suggests in his answer.