Currently, the server running Roundcube needs to have aspell/pspell support. If the server is Linux, the packages you need are aspell, php-pspell and at least one dictionary, for example aspell-em for English.
I do not know how to install pspell on a Windows server, or if it is even available for that platform.
On my RHEL5 server, the exact packages I installed were:
aspell-0.60.3-12
php53-pspell-5.3.3-13.el5_9.1
aspell-en-6.0-3