there's a free reverb-plugin at kvr-vst.com, i don't remember its name, but it uses an almost identical convolution-technique as Altiverb for mac.
just do a search on http://www.kvr-vst.com on 'convolution' and you'll probably find it

;)

You will need to find impulse files. Check out [[url=http://[/URL]="http://noisevault.c…"]Noisevault.com[/]="http://noisevault.c…"]Noisevault.com[/].

sir is cool but has a lot of latency. you can load those impules into the audioflux impulse reverb from voxengo and be good to go..
i use Rverb, 2016 stereo room, Roomverb M2 and Audioflux on my pc with nuendo 2.2
other than those, pick a uad-1 up from yourself