I just went searching for a tight, small little FTP server for Windows.

ZFTP server runs as a system service, small, tight, and, as far as I can tell, performs better than IIS 5's FTP Service.  I'm testing this on a slow, slow Pentium 1 running at 120 MHz (with MMX!) =)  There are marginal improvements in IIS 6... but still no user-based IP access control.

Bloat becomes instantly apparant on limited hardware.  This thing is fast.  Highly recommended. (Oh, and its free!)

http://zftpserver.serveftp.com/

Update: Had a crash with this ftpd last night.  Under heavy load, it seems to lock up.  Can't kill the system service.  Can't end task.  More on this later, when I have time.  For now, I can't even reboot the box, so this piece of software is stuck in memory.  Strange, it didn't do anything weird when I initially load tested it; just bombed out over time I guess.  Hold off on this one until I see what the problem is.  Yes, I'm running the latest build via autoupdate. =)

Update: Latest build has fixed this bug.  Server is excellent.  Highly recommended.  No crashes yet! =)