Seamless 3d needs DirectX 8.1 or up to work. DirectX
is free to download from Microsoft.
Text To Speech (TTS) Version
The TTS version needs .NET 3.5 installed on your computer or it wont run at all.
Even if you only want to use it for modelling it wont run. You do not need the
TTS version to chat but you will need the TTS version if you want to hear chat
voices.
To install .NET 3.5 you download first the dotnetfx35setup.exe which is 2.8 meg. The installation downloads about 50 meg.
It is straight forward to install but takes a while so leave it to do something
else.