Assumptions:
1) It was in this condition when you bought it.
2) The other three windows work fine.
Questions:
a) Is there any noise when you try the driver's side window - like maybe the motor is running but the window doesn't move?
b) How about making the other windows go up & down from the driver's side switch? From their own switch?
Things it might be:
If (a) if might be a small plastic piece has broken and is easily replaced (not common, but not uncommon). Search on the forum for sleeve-lock regulator
if (b) [no personal knowledge - just relating what I've seen discussed over the last year] it seems likely that the switch is bad, the motor itself seldom seems to fail. Do a search, I think some folks have successfully taken apart the switch and cleaned up contacts. Others just replaced the switch. Here's a link to an exploded parts diagram showing the switch:
http://www.subaruparts.com/diag/?mod...category=816-A