Assuming you haven't intentionally added any new hardware or software or recently gotten a cable modem or hooked up to a network, there's a chance you've picked up some nasty spyware.

Run Spybot S&D (shareware) and then, for good measure, run RegCleaner (also shareware -- or maybe even freeware). You'll be surprised how much junk has accumulated on your computer, stealing resources!

Also, a good way to streamline the whole input/output process is to add a second hard drive, dedicated to your video files. It's reasonably cheap and easy and by itself will eliminate the vast majority of capture/make movie problems.