If you don't need anything fancy you can just use the built-in Windows PerfMon. Use the + to add a performance object of Physical Disk, then select one or more of the counters, like Disk Write Bytes/sec, Disk Read Bytes/sec, or Disk Bytes/sec. There is an "Explain" button to tell you a little more about each counter. Copy a large file and watch the graphs. When you click on one of the counters it will show Last, Average, Minimum, Maximum, and duration values update as the file is copying.
This helped me in the past figure out that my Raptor drive was running at 1/3 of normal speed. It turned out to be that one of those older, less sturdy SATA jacks on the motherboard was loose.
You can't transfer any faster than the slowest device.
Maximum (not what you will really get) transfer rates:
(megabytes / megabits per sec)
USB 1.1 - 1.5 / 12
USB 2.0 - 60 / 480
PATA/IDE - 133 / 1064 (UDMA ATA 133, there's also older 66 and 100)
A few others
SATA 1 - 150 / 1200
SATA 2 - 300 / 2400
SATA 3 - 600 / 4800
USB 3.0 - 625 / 5000
Edit: See the List of device bit rates link below from shf301 for a much better chart of storage device transfer rates in bits and bytes.
Some links with screen shots:
adminfoo.net Windows Perfmon The Top Ten Counters
Performance Monitor - Windows Server 2003 Disk counters and logs
(yeah, it's for Server 2003, but it covers monitoring and bottlenecks)