Playback of HD bmp sequences too slow on quad core i7

Question

I have a fairly fast quad-core i7 Win 7 64bit machine with 12GB RAM and primarily use Premiere as an editor for our animated series.  We render 1280x720 30 fps bmp sequences (not interlaced just progressive), which I then import along with the .WAV audio files.  We need to line up sound FX and other editing considerations, but playback on the machine is way too jerky to properly time things.
I assumed this might be improved with PP 5.5 (I have been using PP 2.0 for ever, since it met my needs) but the trial does not reveal that to be the case.  I can, of course, render out to uncompressed AVI sequences to then retime things and that's what I've been doing in PP 2.0, but if that's what I have to do there is little advantage in me upgrading to 5.5.
Is this just the way it is?  I would have thought my machine specs to be adequate to this task, but perhaps not and I'll just have to live with the older program.  Or is there something I should enable to make it work better (I've tried everything I can think of in the program but nothing seems to improve it -- quite frankly, the playback is just as slow as it is on my much older dual-core XP box, also running Premiere 2.0)

Answer 1

Thanks for the responses, guys.
The disc drives are all SATA AV drives with as high a throughput as is reasonably possible.  I had not turned off indexing but I tried that on one drive and it didn't seem to help any (I really couldn't tell any differences).
Yes, of course I have an AV program running, as does anyone nowadays, but with 99.9% of the CPU cycles available I doubt whether it's slowing things down.  When looking at all the cores in Process Explorer, for example, they are nearly as flatline as anything could be.  But if you're suggesting I need to dump any AV as well as, say, even a browser window open to work in Premiere, I'd say it isn't worth it.
That leaves rendering the timeline which, of course, I can do even in 2.0.  I assume the work files involved are at least smaller than rendering out the uncompressed AVIs so that is one advantage, although obviously render times are a PITA compared to timeline playback.  But if that's state of the art nowadays (that is, if most *reasonable* modern systems aren't capable of handing this kind of playback) then so be it.  There's not a real compelling reason for me to upgrade now, then, but that's okay as well.
And, again, I do appreciate the attempt to help (I just wish my system could do this.  Sigh).