MultiScreenshots is a compact tool which automatically captures the screen at intervals of anything from 10 seconds to many hours.
The program can capture the full screen, or a selected area. Images may be saved as JPG or BMP, using your preferred name pattern (sequential numbers or date and time), and you can have the captures stopped after a given time.
MultiScreenshots has some oddities and annoyances. Defining the area you'd like to capture is unusually awkward (you're forced to click in a separate window rather than just draw on the screen). And although it seems to be portable, the program saves its settings to the Registry. There's a button to delete these, but it would be better still if it didn't use them in the first place.
The program should be able to save images as PNG files, too; using BMPs wastes disk space for no real benefit.
Still, if you just want an easy way to take regular full-screen grabs then MultiScreenshots will get the job done.
A basic but effective way to automatically take regular screengrabs