Starting with D-RATS 0.3.2, spell-checking is supported for chat and form messages. In Linux, you should just need to install the aspell package and you're done.  On Windows, you'll need to download and install it manually.  Get it from the following location:

http://aspell.net/win32/

Next, you will need to add the Aspell directory to your path.  To do this, right-click on My Computer, click Properties. Then go to the Advanced tab and click on Environment Variables. In the System Variables group, find the entry for PATH and click Edit. Tack the following on to the end of the string and hit OK:

C:\Program Files\Aspell\bin

You'll know if you got it working because the Check spelling check box in the D-RATS preferences will not let you check it unless it finds the spelling system.