Is this a window regulator failure?
 

Started with the passenger window making grinding noises as it went down or up, then it started getting budged at random spots, but after a little physical push or tug would keep moving.

Now it doesn't work at all, the window stays at its current location, slowly sliding down over the course of a few days. But I can report that the motor works as when you push the button you can hear the motor running, but the window itself does not move.

I opened the door up and tried to move the window down, and it seems that the wire that runs the figure 8 loop through the motor isn't moving when the motor runs.

Do I need a window regulator replacement?

It does sound as if your regulator is kaput since the motor is still functioning.

They do go out on our cars frequently because we're constantly lowering and raising them due to the convertible top operation.

today, I went to put the driver's window down and it started making a grinding / crunching sound. the window is going up/down....but it makes this sound.

at first it sounded like a small rock or pebble had wedged itself into the window track or something..as if it was rubbing on teh window..but i've put the window up/down 5 times and it keeps making the sound.

is this a regulator?

how hard is that to change out?

That sounds exactly like what I experienced at first, a grinding noise for the first few days and then it was accompanied by the window getting stuck randomly.

Replacing the window regulator isn't that bad. I've done both of mine.
Check out this site...

http://www.bombaydigital.com/boxster/projects/window/

Some notes:
Before you loosen the bottom clamps that hold the glass, use a sharpie mark the edges of the clamps so you can put the glass back on at the same location. Same for the mounting positions at the bottom regulator mounting points inside the door. These determine the angle that the window raises- too far in and the window will hit the top outside of the top weather stripping.
And be careful not to tear the thin gray foam moisture barrier- it's pretty expensive.

Replacing window regulator
 

Just want to add my 2 cents. Got my replacement from Sunset Prosche for about $200. Just follow the excellent write-up from
http://www.bombaydigital.com/boxster/projects/window/
Not that hard a job BUT, during installation I reconnected the battery to test out the window function, big mistake! Now the airbag warning light is on continuously. Guess I'll have to get the Prosche dealer to reset that light for me.