Hello mplasse!
On Windows XP with Sibelius 6.2 the sound sets get installed to the following folder:
C:\Documents and Settings\your username\Application Data\Sibelius Software\Sibelius 6
If you have old versions of the VSL sound sets in the folder C:\Program Files\Sibelius Software\Sibelius 6\Sounds, I recommend deleting them.
Best regards,
Andi
Vienna Symphonic Library