This is specifically for Windows 7, but may work with Vista.
Install this:
SpeechPlatformRuntime.msi
Then this:
MSSpeech_TTS_en-GB_Hazel.msi
And then this:
MSSpeech_SR_en-GB_TELE.msi
And finally this reg file:
voices1.zip
Then restart, just to be sure.
That will allow you to use the Hazel voice (included with Windows 8) instead of Anna.
I think there are paid-for alternatives too (IIRC Ivona do one (their Android TTS is awesome, so this should be good too)), but the above works with Windows 7 for free... hopefully for Vista too (the specifications all state they work with Vista, but the registry entries might be different (they are between W7 and W8).