Premiere has gotten some great reviews lately. I use Avid at work. Windows Movie Maker (included in every Windows install) is simple but very effective. Hopefully it has the codecs needed to work with your GoPro footage.
I use Adobe CS5 Premier Pro and Adobe CS5 After Effects. You can do some really uber cool things in them. Not to mention the Photoshop stuff you can do to edit things along with Sound booth . Then don't get me started on the Adobe Encore CS5, the DVD stuff you can make! No wonder Avatar was done with it, but you need a good computer other than a slow laptop with a crappy graphics card to really make it pop.
Windows Movie Maker was included with Windows XP, Vista and 7. You can also get Windows Movie Maker (now called Windows Live Movie Maker) as a free download from the Microsoft website. I believe that WMM is good enough for most people. If you are using a codec that WMM does not support you can probably find a conversion application someplace. You best bet is to use something that is recommended by a computer magazine. Some free programs include a lot of bloatware, be careful to avoid installing that when you install the program.
I have been playing around with video editing for many years now. During this time I principally used Pinnacle (now Avid) Studio through about 5 different versions. I have also used Premier Elements and most recently I have started using Edius Neo. Some thoughts:
1. Video editing is very time consuming and HD can be very demanding on your hardware if you do anything that requires extensive re-rendering (such as changing formats or have fancy effects). The rule of thumb that we used in training video preparation many, many years ago was one hour of editing per minute of completed video. OK, this was with storyboarding and doing a proferssional level of presentation. But still: you get the drift. On top of this: if you have a less than cutting edge computer, rendering as mentioned above can easily take like 5 - 10 minutes of rendering per minute of completed video.
2. Unless you are exceptionally commited to making videos, to my mind fancy features like key framing, elaborate effects etc is redundant. Accepted that I am a very simple soul, but these days I find that I pretty much pull in clips, trim them to suit, perhaps do some fades between scenes, maybe some music/voice over and maybe some lightweight titling. That's it. I personally feel that all kinds of special effects frequently detracts from what you are actually trying to show. KISS works for me.
3. Perhaps (probably?) because I had been a long term Studio user and gotten set in that workflow, I found Premier cumbersome. Powerful, but cumbersome. Note that the learning curve on video editing can be pretty daunting, especially if you want to use more of the more powerful functionality that the good packages like Premier provides. It takes a lot of time to become comfortable with (or even remotely/semi proficient in using) these features.
4. The only reason that I bought Edius Neo is because it can accept any number of HD formats thrown together in one video and export it to numerous formats without blinking. The reason for needing this is that we have like 3 digital still cameras and one good HD video camera: all of them do HD video (720P to full 1080 60P) in different formats and I wanted something that could digest all of ths without choking.
All of this just to lend my strong support to the earlier suggestions to use something straight forward (and FREE!) like Movie Maker. This would enable you to do all the basics without spending a bunch of money. It couild quite possibly even meet your longer term needs but, if not, at least you would have learned the basics and it should make the decision as to what to get once you decide to upgrade to "better" software a lot easier.