Did you update all of your drivers? Soundcard, Video, motherboard etc.? Windows 7 is no doubt superior. I moved up from XP64 to Windows 7 with no issues at all, but I also made sure I had all the latest drivers. I also assume you're using the latest versions of the Vienna Software?
Do you have all of the latest Sibelius updates?
If you have all of the above, try adjusting your latency on your soundcard and see if you can get the clicks to go away. Also make sure you don't have any Anti-Virus etc type software running.
Maestro2be