I've got Win7 too.
So,
Download the CDM20814_Setup.exe file (this is the FTDI 232R DRIVERS XP-Vista-7) and run it.
from here http://www.mediafire.com/?14c462x4kz4dz3y or wherever you find better.
Then all u need to find out on which COM port your USB device is being set (i.e. COM,COM2,COM3 etc).
Having the SIO2USB connected (without being attached to Atari), open AspeQt
Go to Options
Write the appropriate COM port (on your right)
Select the DSR as Handshake method (on your right)
Select a low speed for the baud rate, say 19200 (1x)
Press Ok
Now the console at the bottom should print the following:
Serial port speed set to 19200.
Emulation started through standard serial port backend on 'COM3' with DSR handshaking.
That's it!
Hope it helps.
Had the same issue before, and people helped me too!