Your problem is the external drive. That is never a good idea.
Instead of buying a new computer you could of solved it with putting an internal drive in.

I'm going to chime in here because of my very personal and unsuccessful experience over the last six months trying to edit HD with PP. It's analogous to a Roach Motel. Video checks in, but it can't check out. Oh yeah, then it crashes and may corrupt your project but you have no way of knowing because there's no way to know.

I've edited in both HDV and Cineform. The crashes are too numerous to get anything done in a timely manner. And no... my system is no slug. Quad Core 6600, selective startup, all the Adobe tuning tips. It won't successfully export but once for every four of five tries. About six hours on the phone with Adobe helped, but didn't resolve. Not for their lack of effort. Cineform support was virtually useless.

Oh, by the way I've tried rendering output to Cineform, H.264, wmv, QT, DV, MPEG-2. They will all fail more often than not. And one more thing. This happens on not one, but two Vista machines. An older XP dual core laptop is much happier, but not always.

This rant has a purpose, other than to lower my blood pressure. It's to save you grief and wasted time. Debugging HD in Premier can be a career. The only way I'll try HD again is if I have time and resources to somehow stress test the system over a period of weeks, editing and rendering all the while.

In reverting to DV, I was able to fully produce a 97 minute auto racing piece (edit, cc, multi-cam, titles, graphics and sweeten) in 1.5 days. The previous program (similar content, length and production elements) took eight days to finish and then the only way I could get any footage out was to down-res by exporting each segment to DV. So for me, it's SD.

Maybe it'll work in CS4 or Vista Service pack 9.

I have been editing HD on Premiere for almost 3 years now with great success. The most important thing is your hard drive. You need internal drives at least 3 SCSI 10,000rpm drives set in raid 0.