Altiverb is Mac only and the company that makes it (www.audioease.com) has no plans for a PC version or for releasing their impulses for other plugins (which is a mistake IMO).
Soundwise, there is no difference between Pristine Space and the much more expensive IR1 from Waves. However, Waves has a good bundled impulse library and recently has started a new website for sharing impulses with IR1 owners: www.acoustics.net.
If you can afford it, I would go for IR1 on the PC. If that's too expensive, get Pristine Space and try the impulses from NoiseVault. There is also a free convolution reverb, SIR. I'm not sure about its current status or whereabouts.
I don't think you need special tricks right away with a good impulse for orchestral music. You will need to learn a few things about distance, early reflections, pre-delay etc. Just make sure to check this forum and other forums about this subject.
http://community.vsl.co.at/viewforum.php?f=4&sid=18b8ac4a60cefc65ef070c1d6fccfaa5 - the VSL forum on Mixing and PostPro
http://www.northernsounds.com/forum/forumdisplay.php?f=38 - the NS forum on Reverb/Impulse technologies
http://www.sanctusangelis.com/forum/viewforum.php?f=39&sid=7299661c80773078fe2d3e7b7824ce7d - The V.I. forum on Mixing and Post Pro
http://www.noisevault.com/elephanttalk/viewforum.php?f=1 - The Noise Vault forums on Impulse Reverb
http://www.voxengo.com/phorum/list.php?f=11 - The Pristine Space Support forum
http://waves.org/6/ubb.x - The Waves general support forums
Kind regards,
Peter
www.PeterRoos.com